The sewerage systems which carry the sewage to the site of treatment, or point of discharge, are of two types. Foul sewers carry only domestic and industrial effluent. In areas serviced in this way there are entirely separate systems for the collection of storm-water which is discharged directly to natural water courses.
However, in older towns and cities considerable use has been made of combined foul and storm-water systems. The use of combined sewage systems leads to very significant changes in the flow of sewage during storms.
However, even in foul sewers significant changes in the flow occur due to variations in the pattern of domestic and industrial water usage which is essentially diurnal, and at its greatest during the day. Infiltration will also influence the flow in the sewage system. Although a properly laid sewer is watertight when constructed, ground movement and ageing may allow water to enter the sewer if it is below the water table. The combined total of average daily flows to a sewage treatment works is called the dry weather flow (DWF).
The DWF is an important value in the design and operation of the sewage treatment works and other flows are expressed in terms of it. DWF is defined as the daily rate of flow of sewage (including both domestic and trade waste), together with infiltration, if any, in a sewer in dry weather. This may be measured after a period of 7 consecutive days during which the rainfall has not exceeded 0.25 mm.
The DWF may be calculated from the following formula –
DWF = PQ + I + E
where, P = population served
Q = average domestic water consumption (ld–1)
I = rate of infiltration (ld-1)
E = volume (in litres) of industrial effluent discharged to sewers in 24 hours.
Objectives and Criteria of Sewage Treatment:
The objective of sewage treatment is to produce a sewage effluent which after varying degrees of dilution and self-purification is suitable for abstraction for treatment to produce a potable supply.
Sewage is a complex mixture of suspended and dissolved materials, both categories constitute organic pollution. The strength of sewage and the quality of sewage effluent are described in terms of their suspended solids (SS) and biochemical oxygen demand (BOD).
The SS are determined by weighing after the filtration of a known volume of sample through a standard glass-fibre filter paper, the results are expressed in mgl–1. Dissolved pollutants are determined by the BOD they exert when incubated for 5 days at 20°C. Samples require appropriate dilution with oxygen saturated water and suitable replication. The oxygen consumed is determined and the results again expressed in mgl–1.
Composition of Sewage:
Domestic sewage contains approximately 1000 mgl–1 of impurities of which about two-thirds are organic. Thus sewage is 99.9% water and 0.1% total solids upon evaporation. When present in sewage approximately 50% of this material is dissolved and 50% suspended.
The main components are – nitrogenous compounds—proteins and urea; carbohydrates—sugars, starches, and cellulose; fats—soap, cooking oil, and greases. Inorganic components include chloride, metallic salts, and road grit where combined sewerage is used. Thus, sewage is a dilute, heterogeneous medium which tends to be rich in nitrogen.
Sewage Treatment Processes:
Conventional sewage treatment is a three stage process including preliminary treatment, primary sedimentation, and secondary (biological) treatment. In addition some form of sludge treatment facility is frequently employed, typically anaerobic digestion.
These treatment processes are intended to remove the larger floating and suspended materials. They do not make a significant contribution to reducing the polluting load, but render the sewage more amenable to treatment by removing large objects which could form blockages or damage equipment.
Floating or very large suspended objects are frequently removed by bar screens; these consist of parallel rods with spaces between them which vary from 40 to 80 mm, through which the influent raw sewage must pass.
Material which accumulates on the screen may be removed manually with a rake at small works, but on larger works some form of automatic raking would be used. The material removed from the screens contains a significant amount of putrescible organic matter which is objectionable in nature and may pose a disposal problem. Typically the material is buried or incinerated and less frequently burnt.
If screens have been used to remove the largest suspended and virtually all the floating objects, then it only remains to remove the small stones and grit, which may otherwise damage pumps and valves, to complete the preliminary treatment. This is most frequently achieved by the use of constant velocity grit channels. The channels utilise differential settlement to remove only the heavier grit particles whilst leaving the lighter organic matter in suspension.
A velocity of 0.3 ms–1 is sufficient to allow the grit to settle whilst maintaining the organic solids in suspension. If the grit channels are to function efficiently the velocity must remain constant regardless of variation of the flow to the works (typically between 0.4 and 9 DWF).
This is achieved by using channels with a parabolic cross section controlled by venturi flumes. The grit is removed from the bottom of the channel by a bucket scraper or suction, and organic matter adhering to the grit is removed by washing with the wash water being returned to the sewage.
Small sedimentation tanks from which the sewage overflows at such a rate that only grit will settle out may also be used. These are compact and by the introduction of air on one side a rotary motion can be induced in the sewage which washes the grit in situ. However, these tanks do not cope with the variation in hydraulic load in such an elegant and effective manner as the grit channel.
To avoid the problems associated with the disposal of screenings comminutors are frequently employed in place of screens. Unlike screens which precede grit removal the comminutors are placed downstream of the grit removal process. The comminutors shred the large solids in the flow without removing them.
As a result they are reduced to a suitable size for removal during sedimentation. Comminutors consist of a slotted drum through which the sewage must pass. The drum slowly rotates carrying material which is too large to pass through the drum towards a cutting bar upon which it is shredded before it passes through the drum.
The total flow reaching the sewage treatment works is subjected to both these preliminary treatment processes. However, the works is only able to give full treatment upto a maximum flow of 3 DWF. When the flow to the works exceeds this value the excess flows over a weir to the storm tanks which are normally empty.
If the storm is short, no discharge occurs and the contents of the tanks are pumped back into the works when the flow falls below 3 DWF. If the storm is prolonged then these tanks will begin to discharge to a nearby watercourse, inevitably causing some pollution. However, this excess flow has been subjected to sedimentation which removes some of the polluting material. Moreover, as a consequence of the storm, flow in the watercourse will be high giving greater dilution.
The raw sewage (containing approximately 400 mgl–1 SS and 300 mgl–1 BOD) at a flow rate of 3 DWF or less and with increased homogeneity as a result of the preliminary treatment processes enters the first stage of treatment which reduces its pollutant load, primary sedimentation, or mechanical treatment.
Circular (radial flow) or rectangular (horizontal flow) tanks equipped with mechanical sludge scraping devices are normally used. However, on small works hopper bottom tanks (vertical flow) are preferred; although more expensive to construct these costs are more than offset by savings made as a result of eliminating the requirement for scrapers.
Removal of particles during sedimentation is controlled by the settling characteristics of the particles (their density, size and ability to flocculate), the retention time in the tank (h), the surface loading (m3 m–2 d–1) and to a very limited degree the weir overflow rate (m3 m–1 d–1).
Retention times are generally between 2 and 6 h; however, the most important design criterion is the surface loading, typical values would be in the range 30 to 45 m3 m–2 d–1. The surface loading rate is obtained by dividing the volume of sewage entering the tank each day (m3 d–1) by the surface area of the tank (m2).
The retention time may be fixed independently of the surface loading by selection of the tank depth, typically 2 to 4 m, which increases the volume without influencing the surface area. Because they strongly influence the value for surface loading selected, the nature of the particles in the sewage is one of the most important factors in determining the design and efficiency of the sedimentation tank.
Four different types of settling can occur:
Class 1 Settling – settlement of discrete particles in accordance with theory (Stokes’ Law).
Class 2 Settling – settlement of flocculant particles exhibiting increased velocity during the process.
Zone Settling (Hindered Settlement) – at certain concentrations of flocculant particles, the particles are close enough together for the interparticulate forces to hold the particles fixed relative to one another so that the suspension settles as a unit.
Compressive Settling – at high solids concentrations the particles are in contact and the weight of the particles is in part supported by the lower layer of solids.
During primary sedimentation settlement is of the Class 1 or 2 types. However, in secondary sedimentation, zone or hindered settlement may occur. Compressive settlement only occurs in special sludge thickening tanks.
Primary sedimentation removes approximately 55% of the suspended solids and because some of these solids are biodegradable the BOD is typically reduced by 35%. The floating scum is also removed and combined with the sludge. As a result the effluent from the primary has a SS of approximately 150 mgl–1 and a BOD of approximately 200 mgl–1.
This may be acceptable for discharge to the sea or some estuaries without further treatment. The solids are concentrated into the primary sludge which is typically removed once a day under the influence of hydrostatic pressure.
Secondary (Biological) Treatment:
There are two principal types of biological sewage treatment:
(i) The percolating filter (also referred to as a trickling or biological filter).
(ii) Activated sludge treatment.
Both types of treatment utilise two vessels, a reactor containing the micro-organisms which oxidise the BOD, and a secondary sedimentation tank, which resembles the circular radial flow primary sedimentation tank, in which the micro-organisms are separated from the final effluent.
The early development of biological sewage treatment is not well documented. However, it is established that the percolating filter was developed to overcome the problems associated with the treatment of sewage by land at ‘sewage farms,’ where large areas of land were required for each unit volume of sewage treated.
It was discovered that approximately 10 times the volume of sewage could be treated in a given area per unit time by passing the sewage through a granular medium supported on under-drains designed to allow the access of air to the microbial film coating the granular bed.
The origins of the percolating filter are present in land treatment and its development was an example of evolution. The second and probably predominant form of biological sewage treatment, the activated sludge process, arose spontaneously and represents an entirely original approach.
This process involves the aeration of freely suspended flocculant bacteria, ‘the activated sludge floc’ in conjunction with settled sewage which together constitutes the ‘mixed liquor’. Activated sludge treatment continues the trend established by the change from land treatment to the percolating filter in that at the expense of higher operating costs it is possible to treat very much larger volumes of sewage in a smaller area.
These units consist of circular or rectangular beds of broken rock, gravel, clinker, or slag with a typical size in the range of 50-100 mm. The beds are between 1.5 and 2.0 m deep and of very variable diameter or size depending on the population to be served. The proportion of voids (empty spaces) in the assembled bed is normally in the range 45 to 55%.
The settled sewage trickles through interstices of the medium which constitutes a very large surface area on which a microbial film can develop. It is in this gelatinous film containing bacteria, fungi, protozoa, and on the upper surface algae that the oxidation of the BOD in the settled sewage takes place. The percolating filter is in fact a continuous mixed microbial film reactor.
Settled sewage is fed onto the surface of the filter by some form of distributor mechanism. On circular filters a rotation system of radial sparge pipes is used which are usually reaction-jet propelled although on larger beds they may be electrically driven. With rectangular beds electrically powered rope hauled arms are used.
The micro-organisms which constitute the gelatinous film appear to be organised, at least near the surface of the filter where algae are present, into three layers (Fig. 4.10). The upper fungal layer is very thin (0.33 mm), beneath it the main algal layer is approximately 1.2 mm and both are anchored by a basal layer containing algae, fungi, and bacteria of approximately 0.5 mm.
However, algae do occur to some extent in all three layers. Beneath the surface where sunlight is excluded and as a consequence the algae are absent, this structure is significantly modified, probably into a form of organisation with only two layers. It has been calculated that photosynthesis by algae could provide only 5% or less of the oxygen requirements of the micro-organisms in the filter.
Furthermore, photosynthesis would only be an intermittent source of oxygen since it would not occur in the dark and algae are often present only in the summer months. Carbon dioxide generated by other organisms in the filter might however increase the rate of photosynthesis. It has been proposed that algae derive nitrogen and minerals from the sewage and that some may be facultative heterotrophs. The nitrogen fixing so-called ‘blue-green algae’, really bacteria, are frequently present in filters.
Whilst fungi are efficient in the oxidation of the BOD present in the settled sewage they are not desirable as dominant members of the microbial community. They generate more biomass than bacteria, per unit of BOD consumed, thus increasing the sludge disposal problem. Moreover, an accumulation of predominantly fungal film quickly causes blockages of the interstices of the filter bed material, impeding both drainage and aeration. The latter may result in a reduction in the efficiency of treatment which is dependent upon the metabolic activity of aerobic micro-organisms.
Protozoa and certain metazoans (macrofauna) play an important role in the successful performance of the biological filter, although the precise nature of this role is dependent on the extrapolation of observations made in the activated sludge process, which is more amenable to study.
However, the similarity in the distribution of organisms within the two processes suggests strongly that their roles are the same in both. The protozoa in particular remove free-swimming bacteria thus preventing turbid effluents, since freely suspended bacteria are not settleable.
Certain metazoans may also ingest free-swimming bacteria, but their most important function is to assist in breaking the microbial film which would otherwise block the filter. This film is ‘sloughed off’ with the treated settled sewage. Protozoa (principally ciliates and flagellates) tend to dominate in the upper layers of the filter, whilst the macrofauna (nematodes, rotifers, annelids, and insect larvae) dominate the lower layers.
If film is not removed satisfactorily, frequently as the result of excessive fungal growth; the condition known as ‘ponding’ develops. In this condition the surface of the filter is covered in settled sewage, air flow ceases, treatment stops, and the bed becomes anaerobic. Ponding may also be caused by the growth of a sheet or felt of large filamentous algae principally Phormidian sp. on the face of the filter.
To minimise film production recirculation of treated effluent is often employed. This reduces film growth by dilution of the settled sewage, improves the flushing action for the removal of loose film, and promotes more uniform distribution of the film with depth.
Treated sewage is subject to secondary sedimentation which is similar to primary sedimentation as a result of which the suspended sloughed off film is consolidated into humus sludge and the final effluent discharged to the receiving water.
In the activated sludge process the majority of biological solids removed in the secondary sedimentation tank are recycled (returned sludge) to the aerator. The feedback of most of the cell yield from the sedimentation tank encourages rapid adsorption of the pollutants in the incoming settled sewage and also serves to stabilise the operation over the wide range of dilution rates and substrate concentrations imposed by the diurnal and other fluctuations in the flow and strength of the sewage.
Stability is also provided by the continuous inoculation of the reactor with micro-organisms in the sewage and airflows, which are ultimately derived from human and animal excreta, soil run-off water, and dust. The reactor of the activated sludge plant is usually in the form of long deep channels.
Before entering these channels the returned sludge and settled sewage are mixed thereby forming the ‘mixed liquor’. The retention time of the ‘mixed liquor’ in the aerator is typically 3 to 6 hours, during this period it moves down the length of the channel before passing over a weir, prior to secondary sedimentation.
The sludge which is not returned to the aerator unit is known as surplus activated sludge and has to be disposed off. In practice the conditions in the aeration unit diverge from the completely mixed conditions commonly used for industrial fermentations and it may be best described as a continuous mixed microbial deep reactor with feed-back.
The design of the concrete tanks which form the reactor is strongly influenced by the type of aeration to be employed. Two types are available, compressed (diffused air) and mechanical (surface aeration) (Fig. 4.12). In the diffused air system much of the air supplied is required to create turbulence, to avoid sedimentation of the bacteria responsible for oxidation.
Surface aeration systems introduce the turbulence mechanically and only provide sufficient air for bacterial oxidation. Both types of system aim to maintain a dissolved oxygen concentration of between 1 and 2 mgl–1.
In the diffused air system the air is released through a porous sinter at the base of the tank and this system is characterised by long undivided channels which may be quite narrow. Mechanical aeration utilises rotating paddles to agitate the surface thereby incorporating air and creating a rotating current which maintains the bacterial floes in suspension.
Each paddle is located in its own cell which has a hopper shaped bottom; this gives the plant the appearance of a square lattice (Fig. 4.12). However, beneath the face of the mixed liquor all the cells are connected forming a channel. In both systems the channels are 2-3 m deep and 40-100 m long.
The success of the activated sludge process is dependent on the ability of the micro-organisms to form aggregates (flocs) which are able to settle. It is generally accepted that flocculation can be explained by colloidal phenomena and that bacterial extracellular polymers play an important role, but the precise mechanism is not known. The significance of flocculation to the success of the process is not the only characteristic to distinguish it from other industrial continuous cultures.
There are four additional and very significant differences – it utilises a heterogeneous microbial population, growing in a very dilute multi- substrate medium, many of the bacterial cells are not viable and finally the objectives of the process, which are the complete mineralisation of the substrates (principally carbon dioxide, water, ammonia, and/or nitrate) with minimal production of both biomass and metabolites are also unique.
The heterogeneous population present in activated sludge includes bacteria, protozoa, rotifers, nematodes, and fungi. The bacteria alone are responsible for the removal of the dissolved organic material, whilst the protozoa and rotifers ‘graze,’ removing any ‘free-swimming’ and hence non-settleable bacteria, the protozoans and rotifers being large enough to settle during secondary sedimentation.
The role of protozoa in activated sludge has been extensively studied; there are three groups involved – the ciliates, flagellates, and amoebae. It is probably the ciliates (Ciliophora) which constitute the greatest number of species with the greatest number present in each species which play the major role in the clarification process. The effect on effluent quality as a consequence of grazing by protozoa is summarised in Table 4.14.
Not only do the protozoa remove free-swimming activated sludge bacteria but they play an important role in the reduction of pathogenic bacteria, including those which cause diphtheria, cholera, typhus, and streptococcal infections. In the absence of protozoa approximately 50% of these types of organisms are removed while in their presence removals rise to 95%. Nematodes have no significant role in the process, whilst the effects of the fungi are generally deleterious and contribute to or cause non-settleable sludge known as ‘bulking’.
Of the principal groups of substrates only one single substrate (cellulose) is included. Each of the groups includes many substrates for example the ‘sugars’ identified in sewage include glucose, galactose, mannose, lactose, sucrose, maltose, and arabinose, whilst the nitrogenous compounds include proteins, polypeptides, peptides, amino acids, urea, creatine, and amino-sugars.
Since bacteria normally only utilise a single carbon substrate or at the most two, this diversity of substrates in part explains the numerous genera of bacteria isolated from activated sludge, because each substrate under most conditions will sustain one species of bacterium.
Moreover as a consequence of the large number of substrates present in the settled sewage the concentration of individual substrates is far less than the 200 mgl–1 of BOD present, perhaps 20-40 mgl–1 for the most abundant and less than 10 mgl–1 for the less common ones. The concentration of each substrate is further reduced in the aeration tank by dilution with the returned activated sludge which is typically mixed 1:1 with settled sewage resulting in a 50% reduction in substrate concentration.
The low substrate concentration means that the bacteria are in a starved condition. As a consequence many of them are ‘senescent,’ i.e., in that phase between death, as expressed by the loss of viability, and breakdown of the osmotic regulatory system (the moribund state) thus the bacterium is a functioning biological entity incapable of multiplication.
Unlike the percolating filter, bacterial growth in the activated sludge process is amenable to the type of description used by bacteriologists for conventional continuous cultures. However, although it is amenable to this type of treatment it inevitably appears to be very different from all other continuous cultures.
The dilution rates (rate of inflow of settled sewage/aeration tank volume) used are invariably low by the standards of industrial fermentations, typically 0.25 h–1, i.e., one quarter of the aeration tank volume is displaced every hour, therefore the hydraulic retention time is four hours.
Although in the conventional single pass reactor the dilution rate and the specific growth rate (time required for a doubling of the population) are identical. That is the state in which the rate of production of cells through growth equals the rate of the loss of cells through the overflow. In the activated sludge process because of the recycling of the biomass the specific growth rate is very much lower than the dilution rate, typically in the range 0.002-0.007 h–1.
Since under steady-state conditions, the bacteria are only able to grow at the same rate as they are lost from the system, recycling them dramatically lowers their specific growth rate and allows it to be controlled independently of the dilution rate.
Under steady-state conditions the specific growth rate is equivalent to the specific rate of sludge wastage (mass of suspended solids lost by sludge wastage and discharged in the effluent in unit time as a proportion of the total mass in the plant) which is the reciprocal of the ‘sludge age’ or mean cell retention time which is typically 4-9 days.
Thus, whilst the retention of the aqueous phase in the system is only 4 h, the retention of the bacterial cells or sludge age is several days. The sludge age (θc) is a value which describes a great deal about the type of activated sludge plant; its purpose, quality of effluent, and bacteriological and biochemical states are all summarised by this term.
The activated sludge process may have up to four phases:
(i) Clarification, by flocculation of suspended and colloidal matter.
(ii) Oxidation of carbonaceous matter;
(iii) Oxidation of nitrogenous matter
(iv) Auto-digestion of the activated sludge.
The occurrence of these four phases is directly dependent on increasing sludge age. Those processes which operate at low sludge ages give rapid removal of BOD per unit time, but the effluent is of poor quality. Plants which have high sludge ages give good quality effluents but only a slow rate of removal. Low sludge ages result in actively growing bacteria, and consequently high sludge production, whilst bacteria grown at high sludge ages behave conversely.
Fig. 4.13 illustrates the relationships between the growth curve of the bacterial culture and the type of activated sludge plant. By operating continuously the activated sludge process functions only over a small region of the batch growth curve, this region is determined by the specific sludge wastage rate. The region selected determines the type of plant and its performance. These are summarised in Fig. 4.13.
This type of process is rarely used and is not applicable to the treatment of municipal sewage but may be of use in the preliminary treatment of some industrial wastes. The bacteria are growing rapidly (exponential phase), thus, the process has the ability to remove a large quantity of BOD per unit of biomass and as a consequence a small reactor may be used which is cheap to construct.
However, because of their high rate of growth, the bacteria convert much of the BOD into biomass, causing a sludge disposal problem; flocculation is limited so additional treatment is essential to remove solids.
High-Rate Activated Sludge:
This shares many features of the previous process; however flocculation proceeds satisfactorily and secondary sedimentation will remove the solids effectively. The growth rate of the bacteria is still high, only carbonaceous material will be oxidised. However, some 60 to 70% of the influent BOD will be removed with a hydraulic retention time of approximately 2 h.
This type of process is probably most frequently used for industrial wastes prior to discharge to the sewers, although it is also used for domestic sewage treatment, perhaps most appropriately where effluents are to be discharged to estuarine waters where standards are less stringent.
Conventional Activated Sludge:
The two previous processes utilise actively growing bacteria in the exponential phase of growth. They achieve the oxidation of carbon compounds utilising an exclusively heterotrophic bacterial population. Conventional activated sludge plants operate in the stationary or declining growth phases utilising senescent bacteria. This very slow growth results in very low residual substrate concentrations and hence low values for effluent BOD.
In addition, plants operating at sludge ages towards the upper end of this range contain autotrophic nitrifying bacteria. These organisms convert ammonia to nitrite and nitrate. This further improves the quality of the effluent since ammonia can exert an oxygen demand but nitrate cannot. In addition to maximising effluent quality conventional activated sludge plants limit the production of new cells.
Bacteria which are growing slowly use much of the organic matter available in the maintenance of their cells rather than in the production of new cells. These features have made conventional activated sludge the most widely adopted biological sewage treatment process for medium and large communities. The rate of oxidation is highest at the inlet of the tank and it can be difficult to maintain aerobic conditions. Two solutions to this problem have been adopted.
With tapered aeration rather than supplying air uniformly along the length of the tank the air is concentrated at the beginning of the tank and progressively reduced along its length. The volume of air supplied remains unchanged but it is distributed according to demand. Alternatively stepped loading may be utilised. This aims to make the requirement for air uniform by adding the settled sewage at intervals along the tank, thus distributing the demand.
This process operates at very high sludge ages exclusively in the declining phase of growth. The retention time in the aeration tank is between 24 h and 24 days. As a consequence, the available substrate concentration is low and the bacteria undertake endogenous respiration (Fig. 4.13) that is respiration after the consumption of all available extracellular substrate.
The result of utilising endogenous materials is the breakdown of the sludge, sometimes referred to as auto-digestion. By this means sludge production is minimised and the small amount of material that must be disposed off is highly mineralised and inoffensive. This type of treatment has been extensively used for small communities, whilst capital costs of such plants are high; operating and sludge disposal costs are very low.
The contact stabilisation process is a variation of conventional activated sludge used for treating wastes with a high content of biodegradable colloidal and suspended matter. The process utilises the adsorptive properties of the sludge to remove the polluting material very rapidly (0.5 – 1 h) in a small aeration tank. The mixed liquor is then settled and passed into a second aeration tank and aerated for a further 5 to 6 h, during which period the adsorbed material is oxidised.
After this the sludge with its adsorptive capacity restored is returned to the contact basin. Although this process requires two aeration tanks, the two are very much smaller than the equivalent single tank, since the mixed liquor suspended solids in the contact basin are typically 2000 mgl–1 and in the second tank (digestion unit) they are about 20,000 mgl–1.
The production of a final effluent, with the minimum BOD value, is dependent upon the complete nitrification of the effluent, which involves the conversion of the ammonia present to nitrate. This is a two stage process undertaken by autotrophic bacteria principally from the general Nitrosomonas and Nitrobacter. Nitrification occurs in percolating filters and activated sludge plants operated in a suitable manner. The first stage, sometimes referred to as ‘nitrosification’ involves the oxidation of ammonium ions to nitrite and follows the general formula –
Two important points are evident from this last formula. Firstly, nitrification requires a considerable quantity of oxygen. Secondly, hydrogen ions are formed and hence the pH of the wastewater will fall slightly during nitrification.
The settled sewage is effectively self-buffering but a fall of 0.2 of a pH unit is frequently observed at the onset of nitrification. In this autotrophic nitrification process, ammonia, or nitrite provide the energy source, oxygen the electron acceptor, ammonia the nitrogen source and carbon dioxide the carbon source. The carbon dioxide is provided by the heterotrophic oxidation of carbonaceous nutrient, by reaction of the acid produced during nitrification with carbonate or bicarbonate present in the wastewater, or carbon dioxide in the air.
Whereas for carbonaceous removal the oxygen requirement is roughly weight for weight with the nutrients oxidised, in the case of ammonia, removal by nitrification requires approximately seven times as much oxygen as is required to achieve the removal of the same quantity of nutrient.
Nitrification significantly increases the cost of sewage treatment since more air is required. Furthermore, because these autotrophic organisms grow only slowly, longer retention periods are also required resulting in higher capital costs. Nor does nitrification result in the production of an entirely acceptable sewage effluent. In areas where water re-use is practised the concentration of nitrate in river waters causes concern. There exists a limit on the concentration of nitrate in drinking water to avoid the occurrence of methaemoglobinaemia (so called ‘blue baby’ syndrome).
As a consequence denitrification is now practised after nitrification in some activated sludge treatment plants. In this anoxic heterotrophic bacterial process, nitrite and nitrate replace oxygen in the respiratory mechanism and gaseous nitrogen compounds are formed (nitrogen gas, nitrous, and nitric oxides). However, this procedure is not part of conventional sewage treatment practice at present.
Both types of biological treatment require sedimentation to remove suspended matter from the oxidised effluent. Tanks similar to those normally employed for primary sedimentation are generally employed, although, at a higher loading of approximately 40 m3 m–2 d–1, at 3DWF, because of the lighter and more homogenous nature of secondary sludge, simpler sludge scrapers are possible and scum removal is not necessary. The association of primary sedimentation tanks and a biological process for secondary treatment, results in a sewage treatment works, as opposed to sewage farms where only land treatment is employed.
As an awareness of environmental pollution, in addition to public health, developed in the fifties and sixties, the term water pollution control works was introduced to describe sewage treatment works, although this change of terminology was merely cosmetic. With the recognition of the importance of water re-use the term water reclamation works has found favour in some areas. Such works frequently apply additional tertiary treatment processes.
Sewage treatment results in the production of a final effluent suitable for discharge to the selected receiving water and one or more sludges which may require treatment prior to disposal.
Different treatment units like primary settling tanks, trickling filters, activated sludge plants etc. will give out sludge. Grit chambers also produce solids, which are mostly inorganic in nature and the quantity is also comparatively much less. Sludge from other units as mentioned above contain putrescible organic substances, pathogenic organisms etc. and unsafe to public health. So due care is required in treatment and safe disposal of the sludge. Nature and quantity of sludge produced vary from one unit to other.
Quality of sludge produced from various sewage treatment units are given below:
(i) Pre-sedimentation sludge – This is frequently known as primary or raw sludge and it is produced by pre-sedimentation tanks.
(ii) Activated sludge – Activated sludge produced is collected through secondary settling tanks. The quantity of sludge thus produced is huge and its handling, treatment and disposal become a big task. Its moisture content will be 99 per cent or higher. And volatile solids will about 70 per cent of dry solid contents.
(iii) Trickling filter sludge – Sludge produced from trickling filter unit is also collected through humus tank or secondary settling tank. The quantity of sludge produced is lesser in quantity in the range of about 5 cu.m. per million litres of sewage treated.
(iv) Chemical precipitation sludge – Water content is usually about 90 to 95 per cent. Though the solid content is higher due to higher suspended solid removal and floe accumulation, the volume of the sludge does not increase effectively due to lower, moisture content.
(v) Sludge volume – As it has been seen that dry solid contents of fresh sludge is comparatively less and moisture content is much higher ranging from 95 to 99 per cent. The volume of the sludge increases mainly due to its water content. Again a small change in moisture content will appreciably reduce the sludge volume.
Sludge Treatment and Disposal:
Sludge treatment and disposal is a facet of wastewater treatment which is often given insufficient attention. Sludge treatment and disposal may account for 40% of the operating costs of a wastewater treatment facility.
Prior to treatment the sludges contain between 1 and 7% solids (they are therefore nearly all water) which are usually highly putrescible and offensive. The sludges are the product of primary sedimentation of raw sewage and the by-product of secondary aerobic treatment of settled sewage.
Primary sludge is particularly offensive, with a pronounced faecal odour and is liable to become putrescent thus causing a nuisance. Secondary sludge consists very largely of bacterial solids. It is less offensive than primary sludge but may still become putrescent. These sludges are sometimes combined during sewage treatment as a consequence of co-settlement of waste activated sludge in the primary sedimentation tanks.
The main aims of sludge treatment are to make it easier and cheaper to dispose of the sludge’ consistent with minimising any nuisance or adverse effects on the environment generally. A wide range of treatment processes and disposal options have been used, although, recently the cost of energy has reduced the numbers currently employed because of economic considerations.
The most convenient and economical method of disposal at any given site depends on a number of factors. Treatment of sludge is frequently influenced by the final disposal option selected. If sludge is to be disposed to sea from a works where the sludge may be pumped directly to the disposal vessel then little treatment is required.
Should the treatment works be close enough to the sea to make that type of disposal feasible, but not close enough to allow direct pumping to the disposal vessel, then economies in transport costs may be achieved by utilising some type of treatment process to thicken the sludge and reduce its water content prior to transport to the disposal vessel.
If sludge is to be disposed off to land it is desirable to reduce transport costs, since the sludge will have to be spread over a wide area, and, in the case of treatment works in urban locations, transported a significant distance to reach suitable land.
At present sludge produced is disposed to land, sea and is incinerated. Of the sludge disposed to land is applied to agricultural and horticultural land and for land reclamation and land fill.
The processes available for sludge treatment include – thickening by stirring or flotation; digestion, aerobically or anaerobically; heat treatment; composting with domestic refuse; chemical conditioning with either organic or inorganic materials; dewatering, on drying beds, in filter presses, by vacuum filtration, or centrifugation; heat drying; incineration in multiple hearth or fluidised bed furnaces and wet air oxidation.
It is not feasible within this presentation to deal with all these processes in depth and the following is confined to the predominant sludge treatment process, anaerobic digestion, mechanical dewatering, and the most frequently utilised disposal option, that to agricultural land.
During anaerobic digestion the organic matter present in the sewage sludge is biologically converted to a gas typically containing 70% methane and 30% carbon dioxide. The process is undertaken in an airtight reactor usually equipped with a floating gas collector.
Sludge may be introduced continuously, but more frequently is added intermittently, and the digester operates on a ‘fill and draw’ process. The methane produced is generally utilised for maintaining the process temperature, heating and power production by combustion in dual fuel engines which use oil in the absence of methane.
Methane production is only significant at elevated temperatures, when 1 m3 of methane at STP is produced for every 3 kg of BOD degraded. Digesters are characterised by the temperature at which they operate, those in which gas production is optimum at 35°C are described as ‘mesophilic’ whilst those in which gas production is optimum at 55°C are ‘thermophilic,’ these terms describe the temperature preferences of the bacteria undertaking the process.
Heat exchangers are used to transfer heat from the treated sludge to the influent sludge. The additional heat is provided by the combustion of methane. To minimise heat loss digesters are frequently surrounded by earth banks to provide insulation. For efficient operation the digester requires a mixing system which may be mechanical or utilise the gas produced in the process to provide turbulence.
The result of anaerobic digestion is to reduce the volatile solids present in the original sludge by 50% and the total solids by 30%. In addition the unpleasant odour associated with the raw sludge is drastically reduced. During the 20 to 40 days required for digestion the sludge is stabilised and emerges with a slightly tarry odour.
Traditionally, anaerobic digestion has been considered a two stage process, a non-methanogenic stage followed by a methanogenic stage. The non-methanogenic stage has also been referred to as the acid forming stage since volatile fatty acids are the principal products. However, it is now recognised that the first stage may include as many as three steps.
The first, involving the hydrolysis of the fats, proteins and polysaccharides present in the sludge, produces long chain fatty acids, glycerol, short chain peptides, amino acids, monosaccharides, and disaccharides.
The second step (acid formation) involves the formation of a range of relatively low molecular weight materials including hydrogen, formic and acetic acids, other fatty acids, ketones, and alcohol. It is now recognised that only hydrogen, formic acid and acetic acid can be utilised as substrates by the methanogenic bacteria.
Thus, in the third step compounds other than hydrogen, formic acid and acetic acid are converted by the obligatory hydrogen producing acetogenic (OHPA) bacteria. Some bacteria are able to undertake both steps 1 and 2 and produce hydrogen, formic acid and acetic acid which therefore do not require step 3.
These stages are summarised in Fig. 4.15. Once in operation, with reasonable retention times and volatile solids loadings, routine operation of digesters must include careful monitoring of certain parameters which are used to indicate whether the process is about to fail. The main parameters are volatile acids and hydrogen ion concentration (pH).
Anaerobic digestion is quite sensitive to fairly low concentrations of toxic pollutants, such as heavy metals and chlorinated organics, and to variations in loading rates and other operational aspects. If the balance of the process is upset it is most likely that the methanogenic organisms become inhibited first. This results in a build-up of the intermediate compounds at the stage immediately prior to methane formation. These intermediates collectively are called volatile fatty acids.
They include formic, acetic, and butyric acids and can be monitored to determine the state of the process. The volatile acids are important because of their acidic nature. Normally digesters operate in the pH range of neutrality. They also have some resistance to pH change. High concentrations of the volatile acids can cause a reduction in pH sufficient to inhibit bacterial activity to the extent where irreversible failure of the process occurs.
Because of their capacity to resist changes in pH, volatile acid concentrations can build up to significant levels before pH change occurs. Therefore, they can act as an early warning indicator of impending process failure. Normal levels of volatile acids are 250-1000 mgl–1.
If they exceed 2000 mgl–1 this could lead very quickly to failure: if they exceed 5000 mgl–1 failure is almost inevitable. The adverse effects of volatile acid build-up can be rectified by the addition of lime to restore the balance between acidity and alkalinity.
Anaerobically digested sludge is frequently further dewatered in lagoons prior to disposal. Supernatant liquors are pumped from the surface of the lagoons to the head of the works for treatment.
Dewatering of Sludge:
One of the major objectives of sludge treatment is to reduce the water content. The advantages of this are two-fold. First it reduces the volume of sludge to be handled, which can very often lead to savings in transport costs, and secondly it can improve the physical properties of the sludge making it easier to handle. Sludge can be dewatered in two main ways. Either it can be allowed to dry out naturally or it can be dewatered by forcing the water out mechanically, typically, by either pressure filtration or vacuum filtration.
If sludge is to be dried naturally it is usually spread in layers up to about 2-3 cm thick in special drying beds. These very thin layers permit water loss both by evaporation from the surface and drainage. The sludge lies on a layer of fine ash, over a layer of coarse ash, under which are laid underdrains. The liquor which drains off the sludge goes to a central sump. From there it is pumped back to the main treatment works to undergo aerobic treatment.
After a period of about 2 months the solids content increases to about 25% and the sludge can be dug up. This can be done manually although mechanical scrapers which transfer the sludge onto a moving conveyor are sometimes used. Because the sludge is spread in such thin layers, a large land area is required for drying beds. A drying area of about 0.3 m2 per head of population is normally required.
Sludges are usually difficult to dewater and their dewaterability can usually be improved by the use of conditioners and this is normally the practice if mechanical dewatering is to be employed. Frequently aluminium and iron compounds are used, such as aluminium chlorohydrate and ferrous sulphate, or alternatively organic polymers called polyelectrolytes may be used. It is not certain how these work but it is probable that they react with the surfaces of the small sludge particles which would appear to be the cause of poor dewatering.
Following conditioning, sludge may be dewatered by pressure filtration. The sludge is pumped at a pressure of 700 kPa into a cloth lined chamber – at this pressure requires between 2 and 18 hours to form a cake of 25-30% solids. The process can be operated either in batch or continuous manner.
Alternatively, vacuum filtration may be used; this is invariably a continuous process. A drum, containing several internal segments, revolves on its horizontal axis partially submerged in sludge. A partial vacuum inside the drum causes the sludge to adhere to it in a thin layer and as it rotates out of the wet sludge the water is sucked out. A scraper separates the dried sludge from the outside of the drum.
At the point where the scraper is positioned, the vacuum in the corresponding segment inside the drum is released, aiding the release of the sludge from the outside. The pressure difference between inside and outside is 70 kPa, which is only about 10% of the pressure attained in pressure filtration. Hence, the dried sludge typically only contains 15-20% solids. A diagram of a vacuum filter is shown in Fig. 4.16.
Disposal of Sewage Sludge to Land:
The practice of disposing sewage sludge to land has several potential benefits. Good practice in sewage sludge disposal, whether to land, to sea, or by incineration, involves striking a balance between economic constraints and the avoidance as far as possible of adverse effects on man, animals, and the environment.
Low cost is not always compatible with limited adverse environmental effects and sometimes compromises have to be reached. The best option of treatment within a disposal method may change with time due to changing costs, improved knowledge of treatment processes and environmental effects, experience and research.
Disposal of sewage sludge involving application to agricultural land has the benefit of resource recovery and the value of the nutrients utilised should be taken into account in assessing the minimum cost to the nation.
Disposal of sludge to land can have greater environmental impact than other options and in assessing likely benefits and potential adverse effects consideration should also be given to amenity, formal and informal recreation and wildlife. Where ever it is economically justified and environmentally acceptable sewage sludge should be utilised on agricultural land in accordance with recommendations.
Most of the sludge used in agriculture (about 97%) receives some form of treatment. Nearly half of it is anaerobically digested. Slightly more goes to general arable land than to grazing land. Grass grown for hay or silage is particularly suitable to receive sludge because it reduces the risks of transmission of disease. Fields used for forage crops and cereals are also suitable to receive sludge. Both dried and liquid sludges are applied to land, the latter from tankers by spraying.
Arable land can be ploughed, following the application, which speeds up incorporation of the sludge and can reduce any odour problems. However, when it is applied, even spreading is important to prevent localised ‘hotspots.’ Liquid sludges may also be injected directly into the soil giving uniform application and almost complete elimination of odour problems.
Ideally, disposal sites should be well away from housing, but have good access. The risk of leachate or run-off contaminating groundwater or surface streams must be carefully considered. If the sludge is to be spread by spraying, care should be taken not to allow drift, especially in windy conditions.
The application of sludge to land may help to slow down the decline in organic matter in soils ‘under modern farming methods, leading to improvements in water holding capacity, porosity, and aggregate stability. The main value of sludge as a fertiliser lies in its nitrogen and phosphorus contents. However, much of the nutrient content may be in organic forms, and thus, be unavailable to plants until mineralisation occurs.
Although from an economic point of view it may be desirable to apply dried sludge to land significant quantities of available forms of the nutrients may be lost during drying. Liquid digested sludge may contain upto 10% (w/w) of nitrogen but only a fraction of this may be in available forms. For the purpose of calculating the available nitrogen in sludge, it is assumed that 85% of the total nitrogen in liquid digested sludge and 33% of that in dried sludge is available to crops during the growing season.
Since most agricultural soils are deficient in nutrients, fertilisers nearly always have to be added. However, sewage sludge is deficient in potassium and therefore cannot fulfil complete fertiliser requirements.
Liquid sludges consist mainly of water. Some farmers value sludge solely for its water content which can often help to overcome irrigation problems during dry weather.
The potential hazards from the application of sludge to land are protozoal, viral, bacterial, and other pathogens, which are present to the greatest extent in untreated sludges, persistent toxic organic compounds, and toxic heavy metals.
Due to these hazards salad or other crops which may be eaten raw should not be sown until one year after the application of treated sludge. If treated sludge is applied to pasture, animals should not be grazed until 3 weeks after the application. In the case of dairy cattle, whose milk is not to be pasteurised, the period of delay should be 5 weeks.
Heavy metals are of particular concern because they may be detrimental to crop growth or mobilised through the food chain. In many countries there exist guidelines designed to maintain the addition of sludge to land within safe limits.