This article throws light upon the designing and treatment of fourteen main industrial effluents. Some of the industrial effluents are: 1. Electroplating Industry 2. Steel Industry 3. Cane Sugar Industry 4. Paper Industry 5. Pesticides 6. Treatment of Tannery Waste Waters by Stabilisation Pond Method 7. Cyanide Removal: A Case Study of Operational Experience and a few others.
Treatment of Industrial Effluents:
- Treatment of Effluents from Electroplating Industry
- Treatment of Effluents from Steel Industry
- Treatment of Effluents from Cane Sugar Industry
- Treatment of Effluents from Paper Industry
- Treatment of Effluents from Pesticide Industry
- Treatment of Tannery Waste Waters by Stabilisation Pond Method
- Cyanide Removal: A Case Study of Operational Experience
- Phosphatic Fertilizer Waste Water Treatment Employing Lime and By-Product Phosphogypsum
- Treatment and Disposal of Wastewaters from Synthetic Drugs Plant
- Treatment of Wastes from Moped Industry
- Characterisation & Treatment of Synthetic Textile Mill Waste Waters
- Treatment of Distillery Waste by Up-Flow Anaerobic Filter
- Biological Heavy Metal Depollution Technique
- Treatment of Liquid Effluents from a Titanium Dioxide Pigment Plant
1. Treatment of Effluents from Electroplating Industry:
Generally the plating wastes have the following composition:
The recommended method for the removal of wastes is as follows:
Separation and drying of solids – The cyanide effluent can be treated by alkaline chlorination in one reactor, the chromium by ferrous sulphate reduction in another reactor and the two treated effluents, mixed together along with acid effluents containing other toxic metals in a third reactor to precipitate the heavy metals at a pH of 8.5 and above.
The metals are treated with FeSO4 to reduce Cr (vi) ions and others are precipitated by adjusting pH. The sludge containing metal precipitates may be dried on sand beds and disposed of on fallow land as a filling material of flow sheet combining all these operations is shown ahead (ISI: 7453 – 1974).
2. Treatment of Effluents from Steel Industry:
Iron and steel industry leads to discharge a large quantity of suspended and colloidal matter. They reduce the penetration of sunlight and reduce photosynthetic activity of micro-organisms of polluted water bodies. They also lead to heavy siltation of streams and lakes and affect the flow and life in water bodies.
The average chemical characteristics of by-product coke oven effluents consist of:
(1) Dissolved solids
(2) Cyanide 12 – 50 mg/1
(3) Phenol – 140 – 150 mg/1
(4) Sulphide – 30 – 40 mg/1
(5) C.O.D. – 1400 – 2000 mg/I
(6) B.O.D. – 700 – 1200 mg/1
(7) Total alkalinity as CaC03 – 900 – 1500 mg/1.
The recommended flow sheet for treatment of phenolic and other wastes is given (ISI: 8073 – 1976).
3. Treatment of Effluents from Cane Sugar Industry:
The characteristics of combined effluents from sugar factory can be summarized as follows:
Molasses have extremely high BOD of the order of 900,000 mg/1 hence proper care should be given in the mill for molasses not to spill over into waste waters. A flow sheet for treatment of sugar factory effluent in stabilization pond is shown below (ISI: 4903 – 1968).
4. Treatment of Effluents from Paper Industry:
The characteristics of paper industry waste on average are as follows:
The treatment of effluent is done in two stages:
i. Primary Treatment:
Which is called chemical clarification, and
ii. Secondary Treatment:
Which is called activated sludge process.
In Primary treatment, chemical clarification is done in three stages:
(i) Chemical coagulation with hydrated lime,
(ii) Chemical coagulation with (alum + lime) at pH 10.5 to 11.0 and
(iii) pH adjustment to 6-7. The first and 2nd stages are followed by fluocculation and sedimentation. Thus with primary treatment, we achieve removal of BOD and COD up to 90% respectively.
In Secondary treatment, the activated sludge process is capable of converting most organic waste (soluble and insoluble) into more stable inorganic forms or to cellular mass, resulting into highly reduced BOD and COD values.
The process is performed in presence of heterogeneous microbial culture composed of bacteria protozoa, rotifers and fungi. After secondary treatment, the increased bio-mass is disposed of and the remainder returned to aeration units. After secondary treatment, DO level is also maintained by air saturation technique.
The flow sheet is shown in the diagram 4:
The dewatered sludge is heated and ash is used to make concrete blocks or brick.
5. Treatment of Effluents from Pesticide Industry:
Pesticides produce organic pollution and give high value of BOD, COD, chlorides, nitrates and sulphates as given in Table I.
Activated carbon treatment can achieve 100 per cent removal of parathion, malathion, 2, 4, D, aldrin, dieldrin and DDT for all levels of pesticides used.
Besides odour and emulsifiers present in the commercial formulation are also removed by activated carbon. However, large carbon doses are required for good pesticide removal if the wastewater volume is large. The treatment can only be economical if an efficient carbon regeneration process is available.
Potassium permanganate can effectively remove heptachlor and 2, 4 DCP over a wide range of pH. Ozone dosages in excess of 4000 mg/1 were required for COD removals of 80 per cent and more for de-hydro-chlorination unit of BHC wastes. DDT can be easily removed by settling and coagulation followed by filtration.
It is difficult to evaluate the chemical treatment costs for pesticide wastes because of their varying characteristics. The cost also depends on the degree of treatment required and number of chemicals employed for the treatment process.
The biological methods commonly employed in pesticide manufacturing waste treatment include high rate trickling filters, activated sludge with extended contact system, aerated lagoons and stabilization ponds.
It has been observed that high flow, high BOD wastes may be treated more effectively and more economically with activated sludge than anaerobic ponds whereas low flow, low BOD wastes are more cheaply (but less effectively) treated in anaerobic ponds.
For concentrated waste streams, semisolid and solid pesticide bearing wastes can be disposed of by burial since possibility of air pollution and restrictions on indiscriminate dumping of wastes in ocean are associated with incineration and ocean disposal respectively. Recent researches are mostly directed towards development of specific organisms which can degrade the pesticides in question.
6. Treatment of Tannery Waste Waters by Stabilisation Pond Method:
Composite waste waters from various processing units as well as combined waste waters excluding the chrome tanning waste waters were collected for characterisation and treatability studies. The waste waters were analysed as per the Standard Methods (1965).
The wastewaters collected for treatability studies were mixed with domestic sewage in different proportions of 1:1, 1:2, 1:3, 1:4, 1:5 and 1:6. The BOD and COD values of the domestic sewage used for dilution were 120 mg/1 and 180 mg/1, respectively. Fifteen litres of tannery waste-sewage mixtures were placed separately in experimental waste stabilization ponds (aquarium tanks) of size 60 cm x 30 cm x 30 cm.
Acclimatized algal cultures were transferred to the experimental stabilisation ponds containing waste-sewage mixture in different proportions. Waste water-sewage mixtures were allowed to stabilise for 18 days under artificial illumination, samples were periodically collected from the experimental ponds and analysed for algae and chemical parameters such as pH, alkalinity, BOD and COD.
The pH and alkalinity of tannery waste-sewage mixtures during treatment in experimental stabilization pond using acclimatized algal culture ranged from 7.0 to 8.1 and 265 to 570 mg/1, respectively.
The BOD reduction in waste-sewage mixtures during treatment in experimental stabilisation pond for 18 days varied from 64 to 97 percent and was brought down to 27 mg/1 from 370 mg/1 at a dilution of 1:6 (Table I).
Studies carried out by Thabaraj et al., (1962) and the report (1980) on treatment of tannery wastes in oxidation pond indicated that at a detention period of 40 days the BOD of diluted tannery waste mixed with sewage in the ratio of 3:1 came down to 228 mg/1 from 738 mg/1 and 218 mg/1 from 802 mg/1 respectively.
Information on the kinds of algae which multiply in tannery sewage mixture with reference to chemical parameters is not available.
In a detention time of 18 days, the COD reduction in waste-sewage mixtures ranged from 60 to 89 per cent (Table I). The higher percentage reduction recorded was 89 at a dilution of 1:6 as was evident from the reduction of COD to 77 mg/1 from 700 mg/1.
The algal counts in tannery waste-sewage mixtures in the experimental stabilisation ponds are presented in Table II. The algal count during stabilisation in tannery waste-sewage mixture varied from 38 x 104 to 176 x 104/ml. The algal count in 1:1 waste-sewage mixture ranged from 41 x 104 to 130 x 104ml.
A maximum count of 176 x 104ml. was recorded on the 12th day in 1:6 waste-sewage mixture followed by 159 x 104/ml in 1:5 dilution on the same day. Total algal count declined in all the dilutions except in 1:1 waste water-sewage mixture from 15th day onward.
1. Chlamydomonas ehrenbergill
2. Chlorella pyrenoidosa
3. Scenedesmus bujugatus
4. Euglena ocus
5. Microcystis aeruginosa
The result clearly indicates that Chlamydomonas ehrenbergii, Chlorella pyrenoidosa, Scenedesmus bijugatus, Euglena ocus and Microcystis aeruginosa grew well in tannery waste water-sewage mixtures. However Chlamydomonas, Chlorella and Scenedesmus became dominant during stabilisation. Maximum number of Chlamydomonas ehrenbergii was observed on the 12th day of stabilisation in 1:3 waste-sewage mixture.
The BOD and COD values recorded on the day when Chlamydomonas ehrenbergii dominated were 228 mg/1 and 504 mg/1, respectively. The pH and alkalinity values on the day when Chlamydomonas ehrenbergii was dominant, were 7.9 and 555 mg/1, respectively. Presumably, the pH has influenced the alga.
This alga declined at pH and above. C. pyrenoidosa showed steady growth in 1 : 1 waste-sewage mixture reaching a maximum of 49 x 104 / ml on the 18th day of stabilisation when the BOD and COD values were 364 mg/1 and 820 mg/1 respectively.
The maximum percentage distribution of C. pyrenoidosa in 1 : 2, 1 : 3, 1 : 4, 1 : 5 and 1 : 6 waste -sewage mixtures was 29, 25, 32, 31 and 28, respectively. C. pyrenoidosa occurred maximum when the pH ranged from 7.8 to 8.0 under declining BOD and COD values. S. bijugatus was dominant in 1:4, 1:5 and 1:6 waste-sewage mixtures with maximum percentage distribution of 34, 46 and 37.
When the pH was 8.1, maximum occurrence of S. bijugatus was on the 15th day of stabilisation in 1: 6 waste-sewage mixtures. The BOD and COD values of this dilution on the 15th day of stabilisation were 48 mg/1 and 126 mg/1, respectively.
Presumably high pH with declining organic load favoured the maximum growth of S. bijugatus. The maximum percentage distribution of Euglena acus in 1 : 1, 1 : 2, 1 : 3, 1 : 4, 1 : 5 and 1 : 6 waste-sewage mixtures was 31, 31, 28, 28, 28 and 23, respectively.
The BOD and COD values, when E. acus occurred to a maximum of 31 per cent in 1 : 1 waste – sewage mixtures, were 600 mg/1, 1,268 mg/1 and in 1 : 2 mixtures, 401 mg/1 and 832 mg/1, respectively. The pH value was 7.4. E. acus declined at pH 8 and above. The BOD and COD values when E. acus occurred to a maximum of 23 per cent in 1: 6 waste-sewage mixture were 122 mg/1 and 315 mg/1, respectively.
It indicates that E. acus population was directly influenced by the organic content and pH in the substrate. On the 15th day of stabilisation in 1: 6 waste-sewage mixture, maximum number of M. aeruginosa was present when the pH was also maximum (8.1). The BOD and COD values were 48 and 126 mg/1, respectively.
Hence treatability studies by stabilisation pond method using acclimatized algal culture indicated that the BOD and COD reduction of the waste water-sewage mixtures during treatment in experimental stabilisation pond for 18 days ranged from 64 to 93 and 60 to 89 per cent, respectively.
The results clearly indicate that tannery waste is amendable for treatment in stabilisation pond in admixture with sewage. The greater the dilution, more efficient was the degree of treatment. Of the six dilutions studied, 1: 6 dilutions gave the best performance with the reference to the BOD and COD removal compared to the other lower dilutions.
7. Cyanide Removal: A Case Study of Operational Experience:
Waste waters predominantly laden with cyanide are generally discharged by a variety of industries including metal product and metal finishing industries such as tools, automobile industries where heat-treatment and plating operations are employed for surface hardening and surface coating of metal products respectively.
The concentration of cyanide in these wastes is far in excess of prescribed discharge limits and hence these wastes are invariably required to be treated for removal of cyanide, before finally discharged. This paper deals with the operational experience and performance results of cyanide removal plants installed for automobile industry.
The cyanide removal plant designed and installed at an automobile industry during 1979 is for the solid and liquid cyanide wastes mainly discharged from the heat treatment plant operations. The quantum of solid and liquid cyanide wastes generated from the industry amounts to one metric ton per month and 7.0 m3/day respectively.
The treatment plant provided at the automobile industry for cyanide removal is based on the well-known process of “alkaline chlorination” involving destruction of cyanide by employing chlorine at a raised pH by means of alkali addition.
Accordingly, the treatment plant consists of units for reaction, alkali and chlorine addition, settling and pH correction. The treatment plant lay-outs for solid and liquid cyanide wastes are shown in Figs. 1 and 2.
The solid cyanide lumps generated to the extent of one metric ton per month are collected and stored from which 50 kg. Is taken a day for processing. Prior to grinding in the ball mill the G.I. wire pieces present in the lumps are manually segregated.
The grinding operation of ball mill lasts for about 30 minutes. The powdered cyanide is then stored safely in closed containers. For each operation which takes about 4 hours in the treatment plant only 10 kg. of powdered cyanide waste is processed.
The cyanide powder of 10 kg is first dumped in the inlet chamber of reaction tank wherein about 4 M3 water is added to dissolve the cyanide powder. The cyanide goes into solution in the reaction tank and the tank contents are agitated by mechanical agitators provided in the reaction tank. After a few minutes of operation of agitator, the pH of the tank contents is noted to regulate the rate of alkali addition.
Addition of alkali is stopped when the pH of the tank contents reaches to the level of about 10.0 and chlorine is fed from the chlorine container by admixing with fresh water.
As chlorine is being added, the pH and residual chlorine of the tank contents are noted as close intervals.
As and when the pH is found to drop down below 8.5, alkali is added to maintain the pH at 8.5 to 9.0. Continuous addition of chlorine by maintaining a pH level of 8.5 to 9.0 with addition of alkali would result in appearance of residual chlorine which indicates completeness of reaction. The pH and free chlorine measurements are carried out at the plant by means of pH and starch iodide paper respectively.
After ensuring complete reaction by carrying out plant operations as stated above, the chlorinators and agitators of the reaction tanks are put off. The contents of the tank are then transferred into the settling tanks for separation of inherent and newly formed suspended matters, if any present in the raw waste.
Although the final pH of waste resulting from the reaction tank is generally lower than 9.0 which is the permissible discharge limit, the pH sometimes is likely to exceed in the event of excess addition of alkali in the reaction tank.
In order to bring down the pH to less than 9.0 provision for acid addition is made at the settling tank before it is finally disposed. As there could be instances of incomplete reaction due to possible operational lapse, provision for chlorine addition is also made in the setting tank. The each batch operation takes about 4 hours.
The alkali and chlorine addition takes about 1/2 hr. The content is allowed to remain in the reaction tank for 1.5 hour to ensure completion of reaction. Then, the content is further allowed to settle in the settling tank for another 2 hours prior to the disposal.
The treatment for liquid cyanide is also provided on the similar lines of solid cyanide waste treatment. Thus the steps involved and the units provided in the liquid cyanide waste treatment plant are identical. However, the pH correction is carried out for settled effluent.
This is to avoid dissolution of metal hydroxide formed during the reaction of cyanide removal. In order to ensure maximum precipitation of metal ions present in the liquid cyanide waste, alkali is added in excess at the final stage unlike in the case of solid cyanide waste which is devoid of metal ions.
8. Phosphatic Fertilizer Waste Water Treatment Employing Lime and By-Product Phosphogypsum:
Phosphoric Acid Plant:
In this plant, finely ground phosphate rock is digested with sulphuric acid to yield phosphoric acid and semi-stable calcium sulphate hemihydrate (CaSO4. 1/2 H2O). The slurry is taken through a series of air cooled crystallizers to separate CaS04.2H20 referred to as byproduct Phosphogypsum (BpG).
The BpG is filtered and the acid product (30% P205) is concentrated to 54% P2Os in the end product The fluorine bearing exhaust gases from the digester, gypsum crystallizer and acid concentrator arc scrubbed with water and dilute caustic. The gypsum cake is either transported out for dumping or slurried and pumped to settling ponds for disdodsal.
Digester duct wash forms the major sources of waste water from PA Plant Dilute waste streams from scrubbers and evaporator barometric condenser after cooling are recycled in scrubbers. Blow-down from the cooling tower is another major source of waste water.
Rock grinding motor and generator cooling water and floor washings are less contaminated but add to the waste flow.
The combined waste water is around 12 m3 per tonne of P205 (100%). The major pollutants are acidity (5440 mg/1 as CaCO3; pH 1-2), solids (9300 mg/1), phosphate – P (590 mg/1) fluoride – F (4000 mg/1), sulphate and some heavy metals derived from the rock phosphate. Figures in brackets are average values.
The DAP manufacture involves neutralisation of phosphoric acid with anhydrous ammonia in agitated neutraliser and in ammoniator – granulator. The DAP granules produced are dried, screened, air cooled and coated with clay materials to avoid caking, before bagging.
Dust scrubber overflow constitutes the waste water from DAP plant and is collected in settling tanks for recovery and is recycled of DAP in the sludge. The supernatant is let out as waste water. Cooling tower bleed and floor washes also contribute to the waste water.
The combined waste water from DAP plant averages 0.5 m3/1000 kg DAP and is neutral to alkaline with high dissolved solids. The phosphate and ammonia concentrations are high due to the loss of DAP into the waste water.
The characteristics of combined waste water from PA and DAP plants are shown in Table 1. The major pollutants in the waste water are acidity, fluoride, phosphate and ammonia. Heavy metals are present in low concentrations. Row composited, combined waste water from PA and DAP plants was used in the study.
Treatment (Suggested by Muthukumar et.al):
A widely accepted method for removal of high fluoride concentration from waste waters is by addition of excess lime to produce less soluble calcium fluoride with solubility of approximately 8 mg/1 fluoride at pH 11.
Multiple processes can be adopted to achieve low effluent fluoride concentration. Lime treatment of waste water with low fluoride (100 mg/1) to pH 11.3 followed by pH adjustment to 6 – 7 with alum and coagulation with polyelectrolyte or hexametaphosphate can achieve 1-2 mg/1 of fluoride in the final effluent.
Use of calcium chloride along with lime can result in 10 – 20 mg/1 of fluoride at pH 8.0 as compared to lime alone achieving the same removal at pH 11 – 12. Removal of fluoride below 1.0 mg/1 can be obtained by passing the effluent through activated alumina beds regenerated with sulphuric acid or through bone charcoal regenerated with sodium hydroxide. Phosphate Removal
The degree of phosphate removal from waste waters by chemical precipitation is a function of:
(i) Initial phosphate concentration,
(ii) Precipitating cation concentration,
(iii) Concentration of other anions in competition with phosphate for the precipitant, and
(iv) pH of waste water.
Alum, ferrous/ferric sulphate and lime are commonly used for phosphate precipitation. The tendency of aluminium and iron salts to hydrolyse in aqueous solution creates a competition between hydroxide and phosphate ions and thus the phosphorus removal, efficiency is pH dependent.
When lime is used as precipitant, at low pH value phosphate is likely to be removed as relatively less stable mono-and di-calcium phosphates. On further addition, lime reacts with the alkalinity in waste water to form calcium carbonate and to release OH– ions, thereby raising the pH. At elevated pH, a more stable calcium hydroxy phosphate, Ca5 (OH)(PO4)3, is formed.
Removal of Fluoride and Phosphate:
Large quantities of fluoride and phosphate bearing acidic wastewaters are released from phosphatic fertilizer plants. Double lime treatment can be established by EPA as the best practicable technology and best available technology.
A two-stage treatment process using chalk (CaCO3) and lime (CaO) is used. In the first stage, the acidic waste water is neutralized with chalk to pH 3.5 – 4.0 allowing a reaction time of 30 minutes with adequate agitation. More than 85% of fluoride can be precipitated as calcium fluoride, with marginal phosphate removal.
A second-stage treatment, of the clarified effluent from the first stage, with lime to pH 8.0 – 8.5 results in the formation of highly insoluble hydroxy apatite, Ca5(0H)(P04)3 and fluorapatite, Ca10F2(P04)6 complexes and thus the fluoride and phosphate concentrations are reduced to 10 mg/1 and 5 mg/1, respectively, in the final effluent.
In a similar study with phosphoric acid, ‘process, cooling water’, use of calcium carbonate alone in the first stage is inefficient to provide adequate fluoride removal and also leads to greatly increased sludge production. It is observed that with increasing lime dose the residual fluoride reached a minimum at a pH value of 5.5-6.0 and the increased with increasing pH, reaching a relative maximum near pH 9.0.
Without an intermediate clarification step at pH levels of 5.5 – 6.0, fluoride is believed to get resolubilised and hence is not removed to equivalent concentrations until extremely high pH values. Further lime addition to the clarified effluent from first-stage reduces fluoride and phosphate to acceptable low levels.
At pH above 6.0, the fluorosilicates get decomposed to give out soluble fluorides and the reactions are generally considered as:
Thus it can be concluded that the increase in fluoride concentration observed above pH 6.0 in the lime treatment process is primarily due to the decomposition of the unstable fluorosilicates precipitated along with calcium fluoride in the first stage.
Fluoride and Phosphate Removal Using Gypsum:
Gypsum (CaSO4. 2H2O) has been tested for its potential to provide calcium ion in precipitating F and P from wastewaters with an aim to make use of the by-product Phosphogypsum (BPG) from PA manufacture as a treatment chemical. In the first series of tests, sodium fluoride (NaF) solutions are treated with varying doses of CaSO4.2H2O for fluoride removal.
In the next series, phosphate precipitation using CaSO4.2H2O is tested with sodium di-hydrogen orthophosphate (NaH2PO4) solutions at different pH values between 4 and 10. The pH adjustments are done with addition of sulphuric acid or sodium hydroxide.
These exhaustive screening experiments produced the following results:
(i) Irrespective of initial fluoride concentration (250 – 5000 mg/1 as F), calcium sulphate di-hydrate at 25% more than the stoichiometric requirement (as Ca2+ ion) removed fluoride as CaF2, down to 20 mg/1 in the treated effluent; and
(ii) Phosphate removals of more than 98% can be achieved with stoichiometric dosages of CaSO4.2H2O at pH values above 8.5.
9. Treatment and Disposal of Wastewaters from Synthetic Drugs Plant:
Wastewaters used in the treatability Studies and their Characteristics:
The factory discharges mainly three types of wastewaters, viz.:
(1) Highly acidic;
(2) Alkaline; and
(3) Condensate wastewaters, as already described in Part I- Wastewater Characteristics.
It is pertinent to mention that wastewater from the manufacture of intermediates contains mainly mineral acids and inorganic salts with sulphanilic acid as the only predominant organic constituent.
Biological Treatability Studies:
Alkaline wastewater up to 20% and condensate wastewater up to 40% concentration can be treated effectively in a batch scale activated sludge unit after the necessary phosphorus supplementation.
The treated effluent in both the cases exhibits less than 100 mg/1 BOD. The BOD removal in both the cases is greater than 90%. Further experiments were carried out with a 1: 1 mixture of the two wastewaters to determine its treatability.
Oxygen uptake rates using well acclimatized seed and the combined wastewater as substrate indicated that for organic loadings of 0.10, 0.15, and 0.20 kg BOD/kg MLVSS/d, there is neither lag period nor any toxic effect on the seed used in the study.
Activated Sludge Studies:
Completely mixed activated sludge studies with proper acclimatized seed and phosphorus supplementation can be carried out. The data reveals that, with the lower two loadings, effluent BOD is less than 50 mg/1 ; and for the highest loading, it is less than 100 mg/1. The average TOC, COD and BOD reductions are around 80%, 80% and 99% respectively.
Based on the results of the studies, activated sludge treatment units along with the existing ETP to meet increased load after expansion have been recommended. After this augmentation, the entire flow of 6000 m3/d of condensate wastewater can be treated either as admixture with alkaline wastewater or separately.
In case of admixture, the ratio will alter from 1: 1 used in the present study to 1: 3. However, condensate wastewater being more biodegradable than the alkaline wastewater, the altered ratio will have beneficial effect on the treatment efficiency.
10. Treatment of Wastes from Moped Industry:
Moped Industry is one of the largest automobile industries in India with the production of about 2 lakhs mopeds per annum and is also estimated that the production will increase to about 4 lakhs in the year 1984 – 85 and 8.5 lakhs in the year 1989 – 90.
In the process of obtaining various finished metal components, a moped factory has to discharge wastewater containing extremely toxic heavy metal and cyanide ions. Therefore, the moped industry like any other metal industry needs provision of adequate wastewater treatment.
Waste Flow & Characteristics:
The wastewater emanating from the processes are let out into two drains. The drain I collects wastewaters from painting plant and hand washings and septic tank overflow from worker’s toilets. The drain II collects wastewater from electroplating and hard chrome plants. The drain I and II then joins and taken outside the factory.
The data obtained during the investigation and analysis of samples collected is reproduced in Table I and II. It is seen from Table I that the wastewaters discharged from degreasing, painting, zinc plating and alkali dipping operations are alkaline and the wastes from acid dipping, chromium plating and pickling operations are acidic.
The analysis results also reveal that the nickel plating waste contains nickel, chromium plating waste contains nickel and chromium and zinc plating waste contains zinc and cyanide.
Based on the total waste flow and concentration of various pollutants present in the waste, the pollution load discharged by the factory has been worked out and presented in Table III.
As per local pollution control regulation, the wastewater discharged from the factory needs treatments to conform to I. S. Standards (1) prescribed for inland surface water.
The comparison of the wastewater characteristics which is presented in Table II with the prescribed standard reveals that the factory wastewater requires treatment for removal of BOD, Suspended solids, hexavalent chromium, nickel, zinc, cyanide and oil and grease.
It is observed that the combined treated effluent will have BOD ranging from 28 to 30 mg/1. A schematic diagram representing the quantity of wastewater and BOD values is shown in the figure 2.
According to Gandhirajan, in order to assess the extent of removal of heavy metal ions from waste-water certain experiments can be carried out in the laboratory with the composite sample collected from drain II.
Since the theoretical minimum solubility for nickel and zinc falls at the pH range of 10 to 11, the precipitation experiment is carried out at two different pH of 10 to 10.5.
A known volume of metals bearing wastewater is added with 1.0 per cent sodium hydroxide solution, to raise pH to 10/10.5, mixed well and allowed to settle. The supernatant is withdrawn and neutralized with one per cent sulphuric acid and then analysed for metal ions. The analysis results are reproduced in Table IV.
It is seen from Table IV that the heavy metal ions concentration in the treated waste except for zinc have fair agreement with the minimum solubility curve and precipitation technology standards.
The higher concentration of zinc in treated waste is attributed due to formation of soluble zinc cyanide complex. In the case of hexavalent chromium, since the same cannot be directly precipitated out by alkali, it is found to be present in the treated waste.
The above study results necessitate the following:
Segregation of zinc plating’s waste and oxidation of cyanide prior to precipitation of zinc with alkali. The complex zinc cyanide can be effectively treated for oxidation of cyanide ion by alkaline chlorination. Segregation of chromium plating waste and reduction of hexavalent chromium to trivalent chromium prior to precipitation as chromium hydroxide.
Based on the treatability study, the following treatment formulation has been arrived:
Segregation of chromium bearing wastewaters along with the other acidic wastes and reduction of hexavalent chromium to trivalent state by addition of ferrous sulphate in acidic medium. The waste is then mixed with nickel bearing wastewater and other alkaline wastes and pH is raised by addition of sodium hydroxide to precipitate the metals. The settled effluent is there after corrected for pH and finally disposed.
The zinc plating wastewater containing zinc cyanide complex is segregated and detoxified for cyanide by alkaline chlorination by employing bleaching powder and alkali. After cyanide destruction, pH is raised to precipitate zinc as zinc hydroxide and allowed to settle. The settled effluent is then de-chlorinated employing sodium sulphite and corrected for pH by addition of acid and finally disposed into drains.
The solid cyanide waste could be completely detoxified by alkaline chlorination which was confirmed by laboratory treatability study. However, the extent of detoxification depends on the presence of complex cyanide especially ferro cyanide.
In a study carried out by Gandhirajan, on destruction of solid cyanide from an automobile factory, a residual cyanide concentration of 0.2 to 2.5 mg/1 was observed. The sludge produced in the above treatment units are pumped to sludge drying bed. The dried sludge is disposed as land fill.
The oil bearing wastewaters are demulsified with sodium chloride and oil is removed by mechanical oil skimmer in oil separator unit.
The painting booth waste which is discharged once in 3 months is let out into solar evaporation ponds. The dried sludge is disposed off as land fill.
The partially treated domestic wastes in septic tank are further treated in an anaerobic up-flow filter for BOD removal to required level.
The Details of Anaerobic Filter:
In anaerobic filter, waste is biodegraded anaerobically by means of slime adhered on the media. The media employed is 1.8 to 2.0 cm dia. stone chips or gravels. The hydraulic loading rate is 1.0 to 1.5 m3 / m2/day and/the BOD removal is 70 to 80 percent.
The size of anaerobic up flow filter employed in this case is 3m x 3m x 1.2m (L. D) and the material of construction is brickwork in C.M. 1 : 4. A flow diagram of the anaerobic filter provided in this case is shown in the figure 3.
The treatment formulation is shown in Fig. 4 and the flow diagram of treatment plant units is presented in Fig. 5.
Treatment Plant Area and Cost:
The area required for construction of treatment plant works out to 625 sq. m. The estimated capital cost and operating cost are presented in Table V.
The investigations carried out at a moped factory producing 120 moped per day, indicate that the wastewaters from plating operation contains zinc, chromium, nickel and cyanide in excess of permissible limits. Further, the heat treatment section is found to dis6harge cyanide ladden solid lumps and quench water.
The treatability study carried out with live effluents necessitates the segregation of zinc bearing plating wastewater and oxidation of cyanide prior to precipitation of zinc by alkali addition.
On the other hand, chromium plating wastewater requires segregation and reduction of hexavalent chromium prior to precipitation of chromium as chromium hydroxide. The treatment plant requires an area of 625 Sq. m. The capital cost of the plant is estimated to be Rs. 4, 93,950/- and the operating cost per annum is worked out to be Rs. 1, 35,250/-.
11. Characterisation & Treatment of Synthetic Textile Mill Waste Waters:
Textile mill operations consist of weaving, dyeing, printing and finishing. Many processes involve several steps each contributing a particular type of waste. Wastes from synthetic fibre manufacture resemble chemical manufacturing wastes and their treatment depends on the chemical process employed in the fibre manufacture.
The principal constituent of the raw materials consists of polyester. In addition, a large variety of chemicals and dyes (Table – I) are used in the processes. Flow sheet is given in figure 1.
The wet processing operations consists of dyeing, bleaching, printing and finishing. The dyeing and printing processes are water intensive while the water usage in the finishing process is minimal. Before dyeing the fabrics are subjected to scouring (removal of size, stains and lubricants etc.) and bleaching. The chemicals like sodium chlorite, formic acid etc. are employed for bleaching.
The dyeing process is carried out by two methods e.g:
(1) By use of certain dye-carriers which are synthetic organic solvents or
(2) By carrying out the process at an elevated temperature and pressure preferably in an acidic medium at a pH range of 1 to 2.
As a result of the above treatment the fibre swells and the dye is transferred from the carrier to the fibre. The dyeing process is followed by after treatment, such as scouring with synthetic detergents and soaps and washing with liberal quantities of water.
Printing operation is carried out manually as well as mechanically both, the former on screen printing tables and the latter on various printing machines.
For printing of nylon fabrics generally acidic and metal complex type of dyes are used whereas for polyester fabrics, disperse colours are used with a non-volatile acid to maintain the pH at the required level. For printing of polyester-viscose blend or polyester-cotton blend reactive dyes are used along-with the disperse dyes.
From the printing section, the batches of fabrics are dried and treated with steam in the different ageing machines. The washing operation of the Hinted cloth is carried out in different ways according to the type of cloth, its quality and printing process.
The finishing process include stretching, drying, and heat setting for which stenters are used. The cloth is treated with a synthetic finishing agent dried through the stenter. The final finishing operation involves appropriate treatment of the cloth to impart a feel and lusture to the cloth before it is sent to the folding section.
The water requirement of the mill is about 5800 m3/day (inclusive of soft water) out of this 3645 m3/day is consumed in Unit-I and 1105 m3/day in Unit-II. About 1050 m3/day is required for other miscellaneous uses. The major wet processing units of the mill are located in Unit-I and hence the water consumption of 5 time greater than that of Unit-II.
It will be seen from above that 5800 m3 of water is required for producing 65,000 metres of cloth which works out to about 90 litres per metre of cloth produced or about 800 litres per kg cloth made. The corresponding figure for a cotton textile mill is 200 – 300 litres per kg of cloth and is considerably lower.
Source & Characteristics of Waste Water:
The wet processes of unit -1 & n, are the main sources of waste waters in the mill. In addition, waste water from water softening plant boiler blow down and sanitary sewage are also part of the waste water discharged from the mill. The waste water contributed by the two units of the mill vary considerably.
In Unit – I all the major wet processing units are housed and hence it contributes about 91 percent of the total volume of the effluents. Unit No. 2 houses the yam preparation and texturizing, the weaving sections and same specific wet processes like yarn dyeing and carbonization and also the various utilities like water softening and steam generation. This unit contributes about 8 per cent of the total volume of waste water.
Table II gives the quantities of waste waters produced from different sources as estimated from the water use pattern as well as by actual measurement in certain cases. As can be seen from this table, wet processes contribute about 90% of the total waste water while sanitary sewage and other wastes account for 7 and 3 per cent respectively.
From the point of view of characterisation, the process may be divided into the following categories:
(i) Waste water from dyeing process such as jiggers, beam dyeing machines etc.
(ii) Waste from printing sections such as screen printing section, rotary printing section, auto printing section.
(iii) Waste water from carbonizing processes.
(iv) Waste waters from washing machines.
(v) Waste waters from utility, domestic and other sources.
(vi) Combined waste waters from Unit I & II.
Table – III presents the hourly variation with regard to the chemical characteristics of the combined waste waters from the wet processes of Unit -1 & II. Because of the wide variation, composite samples were drawn (both 8 hrs. as well as 24 hrs.) from the combined waste waters from Unit -1 & II.
The results of analysis of these samples are presented in Table – IV. It is seen that the total dissolved solids in the combined waste waters from Unit – I (composited) vary between 980 to 1160 mg/1. Suspended solids vary from 120 – 160 mg/1, BOD between 175 – 209 mg/1 and COD between 330 – 680 mg/1.
The COD to BOD ratio lies between 2.8 to 5.5. The characteristics of the combined waste waters from Unit – II (composited) has a BOD of 30.123 mg/1 and COD 210-690 mg/1. The COD/BOD ratio is not only very high but shows a high degree of variation viz. 2.3 to 20.8.
The possible characteristics of the combined waste waters from Unit -1 & II are presented in Table – V. BOD values ranged from 180 – 182 mg/1, COD was 417-631 mg/1 and Suspended Solids 80 – 130 mg/1. pH varied from 7.3 to 8.0 whereas Nitrogen (total) and Phosphate were in the range of 16 – 75 mg/1 and 1.64 – 1.80 mg/1 respectively.
At the time of the studies these two waste water streams were not combined and hence the samples were mixed in the ratio of their flow rates representing the expected composition for characterisation purpose.
(I) Physicochemical Treatment:
According to Nema et al studies have been carried out for COD removal from waste waters discharged from individual dyeing section by chemical treatment with alum and ferrous sulphate. Lime is used for adjusting the pH of acidic wastes.
It is observed that with an alum dose of 500 mg/1, COD reduction in the range of 30 – 46% is achieved whereas in case of ferrous sulphate, COD reduction varied from 10 to 64 per cent for a corresponding dose of 100-800 mg/1.
Chemical coagulation with alum takes place at a pH range of 7.3 to 8.0. For efficient removal of colour and other colloidal organics present in the dyeing processes waste water, the alum dose requirement is observed to be 500 mg/1 and the lime required for pH adjustment is in the range of 500 to 750 mg/1.
The raw waste pH is in the range of 4.6 to 5.6. The lime requirement for a corresponding dosage of ferrous sulphate in the range of 100 to 800 mg/1 is also about the same range of 500 to 750 mg/1. The efficiency of chemical treatment with ferrous sulphate is maximum at a pH of 9.5. The results are shown in Table – VI. The removal of colour also is observed visually.
(II) Biological Treatment:
Experiments are carried out on the treatment of combined waste water by activated sludge and aerated lagoon system respectively. The feed is obtained by mixing the combined wastes from Unit – I & II, in the proportion of their flows. The Nos of 8 litres capacity aspirator bottles are used simultaneously as batch reactors to simulate two systems namely activated sludge and aerated lagoon.
Acclimatized sludge is used in these experiments. Compressed air is used for supply of oxygen into the systems. The MLSS concentration is maintained at about 1500 mg/1 for activated sludge and 750 mg/1 for aerated lagoon system respectively.
From the results of the activated sludge treatment studies it is observed that with a BOD loading rate of about 0.1 kg BOD-kg MLSS/day and detention time of 24 hours, it is possible to reduce the BOD as well as COD of the waste water by about 85 (Table – VIII).
On the other hand, the results of the aerated lagoon studies showed that with a detention period of 3-7 days and BOD loading of 0.022 – 0.051 kg/m3/day, it is possible to reduce the BOD and COD by about 83-90% (Table – VI).
Based on the results obtained the following treatment alternatives may be suggested. The waste water from dyeing section will be treated chemically using alum and ferrous sulphate as coagulants. Lime is used for adjustment of pH. The treated waste will be combined with the other factory wastes before letting it into the biological treatment units.
The biological treatment for the entire combined waste will comprise of the following two systems:
(1) Activated Sludge:
The combined waste will be taken into the aeration tank. The detention period will be 24 hrs. with system’s parameters as 0.1 kg BOD/kg MLSS and 1500 mg/1 MLSS concentration. The expected BOD as well as COD removal would be of the order of about 85%. The effluent after settling will be discharged into water course.
(2) Aerated Lagoon:
In this treatment system the combined waste can be allowed to undergo treatment in the aeration tank with detention period of 3-7 days and MLSS concentration at about 750 mg/1. The BOD loading will be 0.022-0.051 kg/m3 day.
The efficiency of BOD and COD removal will be about 80- 90%. The treated effluent will be taken to a settling tank where from supernatant can be discharged into water course. The two treatment flow sheets are shown in Fig. on next page.
12. Treatment of Distillery Waste by Up-Flow Anaerobic Filter:
Production of ethyl alcohol from sugarcane molasses is one of the important industries in India. There are 180 distilleries in the country, producing around 800 million litres of alcohol every year, generating 120 billion litres of waste. The distillery effluent carries a huge organic load to the tune of 70,000 to 1,30,000 mg/1 of COD and thus possess a formidable problem for its suitable treatment and safe disposal.
Of the different treatment methods attempted, such as concentration and production of cattle and poultry feed, concentration and incineration for recovery of potash and anaerobic lagooning is commonly employed by practically all the distilleries in India.
However, the method has never been able to attain the desired results and in fact, has created additional problems of environmental pollution such as contamination of the ground water, release of obnoxious gases like H2S, making surrounding filthy.
Possibilities of the production of biogas through anaerobic digestion of distillery waste have been worked out by Sen and Bhaskaran. Frostell in this studies on anaerobic digestion of distillery waste reported 99% reduction in BOD and 85% removal of COD with 0.35 m3 production of biogas per kg of COD.
In order to make the method feasible for actual use in the industry, attempts have been made to reduce the retention period to less than 20 days, however, a sudden drop in the pH of the fermenting mass was noticed with consequent failure of the digester as evidenced by a steep fall in the rate of biogas production (0.10 M3/kg COD removed/day) with considerable reduction in its methane content (12%).
In an effort to improve the efficiency of the anaerobic digestion, particularly in case of industrial wastes, lot of work has been carried out using a variety of bioreactors.
These bioreactors are essentially “retained biomass reactors” and include:
(1) Up-flow anaerobic sludge blanket,
(2) Anaerobic fluidized bed reactor,
(3) Down-flow fixed film reactor and
(4) The Anaerobic filter.
The anaerobic filter, initially developed by Young and McCarty and studied further by many others becomes particularly suitable for treatment of liquid wastes, like distillery waste, where most of the organic load is in the form of dissolved substances.
Since the efforts to reduce the retention period of anaerobic digestion of distillery waste in a conventional digester below 20 days resulted in failure, attempts have been made to use anaerobic filter to digest the waste in order to attain quick mineralization in a short time.
Two bench scale anaerobic filters were fabricated using 90 mm diameter PVC pipe (Fig. 1). The filters had a total volume of 91 each and an active volume of 3.6.1. The filters are packed with stone rubble of 25 mm mean diameter as filter medium.
The filters are filled initially and then charged daily with cattle dung slurry filtered over gauze, to remove coarse particles to avoid clogging. The daily charge is so adjusted as to have 30 days hydraulic retention time.
When the filters get well stabilized for methanogenesis, as evidenced by satisfactory methane content in the biogas produced, the daily charge is changed to distillery effluent without any pH correction. In filter ‘A’, distillery waste diluted with water in 1 : 1 proportion is used as daily charge while the filter ‘B’ received raw distillery waste without any dilution.
Experiments are carried out in order to have three different hydraulic retention times namely 30, 20 and 15 days, corresponding to different loading rates as given in Table 1. The duration of the experiments was 60, 90 and 50 days respectively.
Following observations are made during the course of the experiments:
(1) Measurement of pH of influent and effluent and Minimum-Maximum temperature – every day.
(2) Estimation of COD and Total Volatile Solids of influent and effluent- twice a week using Standard Methods.
(3) Analysis of the biogas produced for its methane content-twice a week using gas chromatograph.
It could be seen from the results that in filter ‘A’, which received diluted distillery waste, the COD removal was 77, 7 and 62% and TVS removal was 87, 73 and 60% for 30, 20 and 15 days hydraulic retention time respectively. The methane content of biogas produced was 72, 65 and 60% for respective hydraulic retention times.
In filter ‘B’ which received raw undiluted distillery waste, the COD removal was 72, 60 and 55% and TVS removal was 64, 54 and 44% respectively. The methane content of the biogas produced was 65, 63 and 59% respectively for 30, 20 and 15 days H.R.T.
pH of the effluent in all the six sets remained well on the alkaline side, ranging Between 7.2 and 7.7. Temperature of the fermenting mass was in the range of 24.5 to 31°C during the investigation.
It is observed by Gadre & Godbole that treatment of distillery waste using anaerobic filter becomes a method of choice. It would bring about quick degradation of organic content of the waste thus significantly removing the pollutional load and would simultaneously produce biogas which can partly replace the costly fuel commonly used in the industry.
This partial replacement of conventional fuel and total elimination of use of lime to neutralize the raw waste, hitherto practiced in the lagooning method, could generate some revenue to the industry thus making the method partially fruitful, financially.
13. Biological Heavy Metal Depollution Technique:
The heavy metals are highly persistent and their toxic effects have been known for centuries. Acute concentrations of these will kill the aquatic organisms is reviewed by Alabaster and Lloyd (1982).
In sub-acute concentrations, these metals get gradually intensified in various aquatic organisms and they reach the higher trophic level through the food chain. These accumulated levels lead to ‘ecological backlashes’ or ‘biological magnification’ resulting in the alteration in growth reproduction and maturity as well as in various vital physiological, biochemical parameters.
It has been observed that the water weed hyacinth has two phases of absorption of heavy metals, one, an initial passive phase that extended for 48 hrs. and an another active phase around 72 hrs. Plants in general have two distinct phases of absorption, one an initial passive phase and another an active transport phase.
During the first phase an exchange of ions tend towards a new position to establish an equilibrium between the free space of the tissue and the external medium; during the second phase a continuous absorption of ions, both cations and anions related to the metabolic activity of tissue occurs (Stiles and Cocking, 1969).
In the second phase the uptake is generally characterised by a certain amount of ionic exchange between the outer medium and the tissue. Thus the two ions of a salt are generally absorbed at different rates and often the cation absorption is accompanied by even exosmosis of the anion from the tissue (Stiles and Cocking, 1969).
The common explanation for this type of prolongation of the first phase was probably that the medium has been modified during the first part of the experiment either by exudation from living cells or by leaching from dead cells to render the heavy metal less permeable by some sort of chelation (Nielsen and Anderson, 1970), before the onset of second phase of active absorption.
The problem of bioaccumulation is a fact supported by various investigations. Various plants have been shown to bio-accumulate various metals from water, suspended particles, sediment and through food chain (Patin et al„ 1974; Stratton and Corke, 1979; Rath et al., 1983).
Various sea weeds and algae can absorb heavy metals and radio-active elements (Bryan, 1969; 1976 a, b; 1971; Jones, 1960; Morris and Bale, 1975; Folsom et al„ 1975; Davies, 1975, 1976 and Matkar et al., 1983).
Water hyacinth (Eichhomia crassipes) an uncontrollable plant pest known to grow and thrive in polluted water (Pieterse, 1978) with an excellent capacity to absorb organic and inorganic chemicals (Wolverton, 1981) including various heavy metals (Wolverton et al., 1976; Wolverton and McDonald, 1978 a, b).
Thus the bioaccumulation capacity of hyacinth has helped to reduce the toxic metals involving the above time course of physiological process indicating that third day’s bioaccumulation capacity could be utilised for depollution.
Therefore knowledge of such time course of mechanisms of absorption is an essential factor to be taken into account so that the plant hyacinth can be utilised in depollution methods. The bark of Babul can also absorbs about 95% toxic metals.
14. Treatment of Liquid Effluents from a Titanium Dioxide Pigment Plant:
Titanium dioxide is widely used as a white pigment. It, however, has other uses in ceramics, printing inks, rubber floor coverings, paper and paper boards, plastic and fibres etc. Present day practice for the manufacture of this pigment is based on acid leaching the titanium followed by purification.
Sulphuric or hydrochloric acids may be used for leaching. Accordingly there are two processes called sulphate route and chloride route of TiO2 production, respectively. The sulphate route was the first to be used commercially and is a batch process. Nearly 80% of all titanium dioxide is produced by this process till today.
The chloride route is a continuous one and has some distinct advantages over the sulphate process although it requires a much higher grade of titanium ore such as rutile (about 90% TiO2), or beneficiated ilmenite, whereas sulphate process can directly use ground ilmenite (about 50-70% TiO2).
Modern plants are being built on chloride route because of its simplicity. Moreover, chloride route is environmentally more compatible than the sulphate route.
In India vast deposits of ilmenite are found in the Quilon District in the State of Kerala where a plant for the manufacture of TiO2 following the chloride route is being set up while there already exists another plant following sulphate route.
Table I gives the break-up of waste-water discharge expected from various plants and the characteristics of the same is shown in Tables II, III and IV. It is found that various wastewater streams are predominantly acidic in nature. The chlorination unit of the pigment production plant contributes the highest pollutional load both in terms of flow and pollutant concentration.
Also most of the streams are abundant in heavy metals like Fe (+2, +3), Cr (+3), V, Nb, Zr and Zn. The wastewater is rich in chlorides also and suspended matters like TiO2 fines, FeO, Fe2O3, coke fines as well as silica will be present.
The combined wastewater from the plant is proposed to be treated with lime followed by clarification and sludge disposal. Based on the local condition prevailing at the factory site two alternative sludge disposal methods have been suggested.
The schemes are shown in the flow sheets (Figs. 16 and 17). Since the wastewater flow and characteristics vary stochastically, it is desirable to equalize the wastewater in a tank having a suitable stirring arrangement.
Two tanks each of six hours storage capacity will be adequate. One tank will be working while the other will be a standby unit. Since no settling of sludge is intended at this state, the stirring arrangements will be provided.
Both the tanks will have acid proof lining jointed by acid resistant cement morter. Similarly, the agitator blades and contact parts will need to be constructed with rubber lined mild steel in order to resist the action of the hydrochloric acid.
The wastewater will be continuously pumped by a polypropylene pump to a flash mixer where 10% lime slurry will be added to the wastewater continuously by means of a lime slurry dosing pump. In order to safeguard the downstream equipment from permanent damage due to acidic action of the wastewater a control valve and locking device are proposed to be installed with the lime dosing pump.
The control mechanism should ensure that the wastewater feed pump stops automatically in case the lime dosing is not proper or fails. From flash mixer the wastewater passes over to a reaction tank where reaction between wastewater and lime slurry takes place under a slow agitation of 5 – 10 rpm.
The results of the laboratory experiments indicate that most of the metallic compounds get converted to their hydroxides at or below the pH of 9.0.
It should be pointed out here that the pH should not go beyond 9.0 since otherwise the effluent will need neutralization with acid for final disposal into a water course according to IS – 2490 (Part I) 1974.
In this alternative the lime treated effluent (Fig. 16) is clarified and taken to a polishing pond before discharge to a water course. The settled sludge from the clarifier will be of very large volume and will have a high water content. Therefore, the same is proposed to be dewatered by filtration in a rotary vacuum filter or pressure filter.
The dewatered filter cake may be disposed off as a solid waste. The filtrate will be recycled to the clarifier along with the treated effluent. The sludge may also be dewatered in a sand drying bed. There will be a polishing pond for the clear effluent from the clarifier. Further purification or polishing of the wastewater by way of settling of suspended solids will take place in this polishing pond.
This will in addition act as a buffer and any malfunctioning of the plant will be partly or fully off set by this buffer pond. The ponds will have to be constructed in series parallel cells to facilitate isolation and periodic cleaning. It is suggested that in the last cell of the pond fishes may be grown which will act as an indicator of pollution.
In this alternative (Fig. 17), the lime treated effluent is directly led to a lagoon or pond. The pond will be divided into two parallel streams: each stream will have two cells in series. One stream will be working while the other will be standby or on cleaning cycle.
The first cell of each stream will serve as a sludge setting lagoon where solids will settle and clear effluent will overflow to the next cell before being discharged into the water course.
Arrangements may be made for dredging the filled up lagoon from time to time. The entire lagoon bottom as well as the sides will be given an impervious lining in order to prevent seepage of effluent to the adjoining areas. While desludging the lagoons the sludge may be pumped directly to the ilmenite mining pits.