The following points highlight the top three methods used for sewage treatment. The methods are: 1. Oxidation Ditch 2. Stabilization Ponds (Oxidation Ponds) 3. Rotating Biological Contactors.
Method # 1. Oxidation Ditch:
The method of oxidation ditch is essentially an extended aeration activated sludge process. It was developed for the treatment of sewage for small towns in Netherlands. An oxidation ditch consists of an endless ditch for the aeration tank and a rotor for mechanical aeration of the sewage. The ditch is in the form of a long continuous channel usually oval in plan, (see Fig. 15.1).
The channel may be an earthen channel with lined sloping sides and lined floor, or it may be built in masonry or concrete with vertical sides. There is normally no primary settling tank provided in the oxidation ditch method. The raw sewage after passing through a bar screen is directly fed to the ditch.
The sewage is aerated by a surface rotor (generally a cage rotor) placed across the channel. The rotor entrains the necessary oxygen into the sewage and also keeps the contents of the ditch mixed and moving. They are designed to impart a velocity of 0.3 to 0.4 m/s to the mixed liquor, preventing the biological sludge from settling out.
Cage rotors usually have a diameter of 700 mm and a speed of 75 rpm. Rotors are manufactured in 300 mm intervals upto 4.5 m length. A rotor assembly can be of multiple lengths but it must be supported by intermediate bearings. The cross-section of the ditch depends on the length of the rotor assembly. The ratio of the width of the ditch and the rotor length is usually between 1.5 and 2.8, the larger value being normally used for short lengths of 0.9 to 1.2 m.
The standard oxygen transfer capacity of rotors is 2.8 kg of O2 per metre length at 160 mm depth of immersion. This rotor is found to impart adequate circulation for 120 to 150 m3 of ditch volume per metre length of rotor. Power requirement per metre length of rotor is about 1.35 kW at the r.p.m and immersion depth stipulated.
The volume of the ditch is determined on the basis of the criteria for the extended aeration activated sludge process. The width of the ditch is kept sufficient to accommodate the length of the aerator required to meet the oxygen demand either as a single rotor or as multiple rotors. The depth of the ditch is kept as 1.0 to 1.5 m, and the length of the ditch is kept to give the required aeration tank volume. The ends of the ditch are rounded to prevent eddying and dead areas.
The raw sewage and return sludge are discharged into the ditch upstream of the rotors. The outlet of the ditch is located geometrically opposite to the inlet. The outlet weir should be baffled and should be of adequate length so that the level of sewage in the ditch does not rise excessively and overload the rotors at periods of peak flow.
The oxidation ditches may be classified as:
(i) Intermittent flow type and
(ii) Continuous flow type.
In the intermittent flow type oxidation ditch, shown in Fig. 15.1 (a), no separate settling tank is used but the ditch itself is used for settling. The flow in the ditch remains suspended and the rotors are stopped during a predetermined period. The supernatant is withdrawn through the outlet. The surplus sludge, settled in the ditch, is removed with the help of a sludge trap.
For intermittent operation the cycle of operation consists of:
(i) Filling the ditch with sewage, closing the inlet valve A, and aerating the sewage;
(ii) Stopping the rotor and letting the contents settle; and
(iii) Opening both inlet and outlet valves, thereby allowing the incoming sewage to displace an equal volume of clarified effluent.
In the continuous flow type oxidation ditch, shown in Fig. 15.1 (b), the operation is kept continuous by allowing the mixed liquor to settle in a separate settling tank. The clarified liquid from the settling tank passes over the effluent weir for final disposal. The settled sludge is removed from the bottom of the settling tank by a pump and is returned to the ditch.
The oxidation ditch is operated as a closed system, and the net growth of the volatile suspended solids will require periodic removal of some sludge from the system. Wasting of the sludge, through a connection to the return sludge line, lowers the concentration in the ditch and keeps the metabolism more active.
The kinetic equations are also applicable for the oxidation ditch process, but with necessary modifications in the available data. In any system treating soluble waste, the MLSS are taken as the mass of the active micro-organisms in the reactor.
However, in the present case, unsettled domestic sewage, which is directly applied to the oxidation ditch, consists of a large portion of inactive VSS in it. Hence influent soluble BOD S0 is equal to La minus BOD of SS contained in the influent. For Indian conditions the BOD of SS may be taken @ 0.25 kg/kg of SS. Similarly the BOD of SS escaping with the effluent may be taken @0.45 kg/kg of SS. The microbial mass concentration may be taken equal to 60% of the MLSS, to take care of inactive VSS in raw sewage.
Thus following equations are for the oxidation ditch process:
Method # 2. Stabilization Ponds (Oxidation Ponds):
Stabilization ponds or waste stabilization ponds are open, flow through earthen basins specifically designed and constructed to treat sewage and biodegradable industrial wastes. The term oxidation ponds, often used, is synonymous. Stabilization ponds provide comparatively long detention periods extending from a few to several days.
During this period putrescible organic matter in the sewage gets stabilized in the pond through a symbiotic relationship between bacteria and algae. These ponds may be considered to be completely mixed biological reactors without solids return.
The mixing is usually provided by natural processes such as wind, heat, fermentation, etc., but may be augmented by mechanical aerators or diffused- air aeration units. However, ponds in which oxygen is provided through mechanical aeration rather than algal photosynthesis are called aerated lagoons.
Under many situations in warm climate countries pond systems are cheaper to construct and operate as compared to conventional methods of sewage treatment. They also do not require skilled operational staff and their performance does not fluctuate from day to day. The only disadvantage of pond systems is that they require relatively large land.
The design criteria for the pond systems have been evolved in our country. An extensive study conducted in our country has revealed that, under certain conditions, the degree of treatment that can be achieved in the case of pond systems is as good as that in the case of conventional systems.
Fig. 15.3 shows a typical plan and section of a stabilization pond. It consists of a shallow pit dug below the ground, and surrounded on all four sides by high levees or embankments. It is constructed in impervious soil such as clay. However, if constructed in more permeable soil, it should be properly lined with impervious soil (clay) or with a synthetic material.
Levees are constructed with 2:1 slope inside and 3:1 slope outside, with a top width of 2.5 to 3 m to form a roadway to provide accessibility. The side slopes are riprapped to prevent erosion from water and wind. A minimum free board of 0.6 m is provided. Influent lines discharge near the centre of the pond and the effluent usually overflows in a corner on the windward side to minimize short circuiting.
The overflow is generally a manhole or box structure with multiple-valved draw off lines to offer flexible operation. In the absence of such a structure a simple but effective means of obtaining draw off is to install a sideways tee type discharge pipe (Fig. 15.3).
In the stabilization pond, a wide variety of microscopic plants such as algae, plankton, etc., find the environment a suitable habitat. The waste stabilization action in the pond is due to the combined activity of algae and bacteria. Algae in the presence of sunlight liberate free oxygen by photo-synthetic action.
The oxygen so liberated is utilized by the bacteria which stabilize the organic matter present in the sewage by oxidizing the organic matter. During the process of oxidation, ammonia, carbon dioxide and other substances are liberated which are used by the algal cells.
Thus a symbiotic cycle is established between the bacteria and algae, and this process which is usually known as bacterial-algal-symbiosis results in complete stabilization of organic wastes. When the pond bottom is anaerobic (due to provision of greater depth or due to lack of proper mixing and aeration), biological activity results in digestion of the settled solids.
The pond water becomes super saturated with dissolved oxygen during afternoon because of release of oxygen by the algae in the presence of sunlight. The suspended solids and BOD in the pond effluent are primarily from the algae. Though the BOD reduction exceeds 95% (especially in summer), the effluent does not meet the standard of 30mg/l of suspended solids. Algae suspended in water during summer months generally contribute 50 to 70 mg/l.
This is a serious problem in the pond treatment of sewage. Also the algal cell remains represent an oxygen demand. Hence both suspended solids and algal cells must be removed from the effluent prior to its discharge either by gravity settling, by floatation, by filtration, or by some other means of suspended solids removal.
Advantages and Disadvantages of Stabilization Ponds:
The advantages of stabilization ponds are as indicated below:
(i) Lower initial cost, being only about 10 to 30% of that of the conventional plant using activated sludge process or trickling filters.
(ii) Lower operating and maintenance costs and no skilled supervision required at any stage of construction or operation.
(iii) Quite flexible in operation. Treatment system is not significantly influenced by a leaky sewage system bringing storm water along with sewage, and do not get upset due to fluctuations in organic loading.
(iv) Regulation of effluent discharge possible thus providing control of pollution during critical times of the year.
The disadvantages of stabilization ponds are as indicated below:
(i) Requires extensive land area. Hence the method can be adopted in those areas where land costs are less.
(ii) Assimilative capacity of certain industrial wastes is poor.
(iii) There is nuisance due to mosquito breeding and bad odours. To avoid mosquito breeding the banks of the ponds should be kept clear of any grasses, bushes, etc. Similarly to avoid bad odours the ponds should be located sufficiently far from residential areas. Odours may also be kept under control by avoiding over-loading. However, when a pond gets over-loaded, the algae growth may be stimulated by adding sodium nitrate which is both a plant food and an oxidizing agent.
(iv) If used in urban area, expansion of town and new developments may encroach upon the pond site.
(v) Effluent quality standards of 30 mg/l for suspended solids are not met.
Method # 3. Rotating Biological Contactors:
Rotating biological contactor (RBC) is one of the recently developed biological treatment devices. It has been widely used abroad but not in India for the treatment of both domestic and industrial wastewaters, especially for small and medium scale units. This is a relatively simple device operating on the principle of moving media. It falls under the category of aerobic attached-growth treatment processes. The RBC units can be adopted for small and medium towns.
As shown in Fig. 15.16 the rotating biological contactor unit consists of:
(i) Cylindrical bottomed horizontal flow tank usually divided into an appropriate number of stages which are hydraulically connected. The tank may be constructed of steel, fibre glass, concrete or masonry.
(ii) A series of closely spaced circular discs of polystyrene, polyvinyl chloride (PVC), asbestos cement or any inert light material of high durability mounted on a horizontal shaft. The disc diameter usually varies from 1 to 4 m and thickness upto 10 mm. The discs are spaced 30 to 40 mm centre to centre and are held partially (40 to 60%) submerged in the sewage flowing through the tank. The shaft is rotated at slow speed, normally less than 10 r.p.m.
(iii) A driving mechanism comprising of a motor and a reduction gear.
Each reactor module consists of a tank with circular discs mounted on a shaft driven by motor through reduction gear. Several modules may be arranged in parallel and/or in series to meet the flow and effluent quality requirements.
When the discs, also called bio discs, come in contact with sewage, biomass or biological growths become attached to the surface of the discs and eventually form a slime layer over the entire wetted surface area of the discs. The rotation of the discs causes the biomass to be alternately submerged in the sewage to adsorb organic matter and to pick up a thin layer of sewage and then raised out of the liquid and exposed to the atmosphere to oxidize the adsorbed organic matter or substrate and to allow the sewage film to slide down the biomass.
Thus the rotation of discs affects oxygen transfer and maintains the biomass in an aerobic condition. Further excess biomass growing on the disc surfaces is sheared off and sloughed biomass is kept in suspension by the mixing action of the rotating discs and carried out of the tank along with the effluent.
Both the substrate utilization within the microbial film and the sloughing of excess biomass are continuous processes which help in maintaining a constant thickness of microbial film on the discs. Thickness of biofilm may reach upto 2 to 4 mm depending upon the strength of sewage and rotational speed of the discs.
The basic process flow sheet of sewage treatment system may consist of primary sedimentation following screening and grit removal, aerobic biological treatment in RBC unit and secondary settling for solid-liquid separation of sloughed film from treated sewage. The settled sludge from primary and secondary sedimentation has to be suitably treated and disposed.
Design and Operational Parameters:
Several process parameters affect the performance of RBC as a biological treatment device. Some of the important parameters include:
(i) Hydraulic loading;
(ii) Hydraulic retention time;
(iii) Influent substrate loading;
(iv) Disc rotational speed;
(v) Disc area available for biological growth; and
(vi) Disc submergence.
The hydraulic loading rates vary depending upon the influent substrate concentration and desired quality of effluent, with typical value around 110 litres per day per m2 of surface area of the discs for primary settled domestic sewage. The corresponding organic loading may be 0.022 kg BOD5 per m2 of surface area per day for BOD5 of 200 mg/l for primary settled sewage. The hydraulic retention time of 1 to 1.5 hour can result in 90% BOD removal efficiency. The disc rotational speed usually varies from 2 to 6 r.p.m. The disc submergence is usually between 40 and 60%.
Reductions of 90% in BOD and SS could be expected at detention periods of 1 to 1.5 hours in the disc chamber and about one hour in the settling tank. The energy consumption varies from 0.6 to 1.2 kWh per kg of BOD removed with a loss of head of less than 2.5 cm through the unit. This energy consumption corresponds to about 6.6 to 13.2 kWh per person per year in comparison to 10 to 16 kWh per person per year for other biological treatment units like activated sludge process, oxidation ditch or aerated lagoons.
Advantages of Rotating Biological Contactor:
The various advantages of rotating biological contactor (RBC) are as indicated below:
1. Low food to micro-organism ratio resulting in higher efficiency of organic matter removal.
2. Low hydraulic retention periods minimizing tank volume and capital costs.
3. Low head loss and lower power requirements.
4. Inherent simplicity and low operational and maintenance cost.
5. Ability to resist shock loads.
6. Ability to lend itself to modular fabrication to suit required effluent quality.