One of the major polluting effects of wastewater on streams results from depletion of dissolved oxygen by the action of aerobic organisms in degrading the organic content of the waste. This suggests that one method of removing organic matter from wastewater would be to concentrate the natural aerobic biodegradation process into an engineered system under controlled conditions. This is done by using either aerobic or anaerobic treatment processes, the latter mainly where the organic loading is very high, often as pretreatment to the aerobic processes or for the treatment of the primary settled sludge.
Conventional sedimentation, the major process in primary wastewater treatment, normally removes 60 to 70 percent of the suspended matter containing 30 to 40 percent of the BOD present in municipal waste waters, leaving 150 to 200 mg/L and about 200 mg/L SS in the primary effluent. Discharge of effluent of this quality without exceeding the assimilative capacity of the receiving environment is only possible where very large volumes of water are available for dilution, or where the effluent may be irrigated over a large land area. For discharge to inland streams or lakes, a considerably higher quality is necessary. This calls for secondary treatment, usually in the form of some biological treatment.
The plant nutrients, nitrogen and phosphorus, can lead to the enrichment of receiving waters (eutrophication). Additional treatment of wastewater, tertiary treatment, may be required to avoid this problem, which can result in serious ecological disturbances.
One alternative to all these sophisticated treatment technologies would be to keep sewage on site and let every household take care of its own wastes. While this avoids all the above problems, it creates new ones and could become a serious health hazard if not properly controlled. Sufficient area would also be required to dispose of all products on site. This topic is covered in the second half of this module.
BIODEGRADATION AND BIOLOGICAL GROWTH Successful biological treatment depends on the development and maintenance of an appropriate, active, mixed microbial population in the system. This microbial population may be present as either a fixed film attached to some form of support medium, as in the trickling filter and rotating biological filter processes, or a suspended growth, as in activated sludge processes and anaerobic digestion. Organic waste matter is used as a food source by the microbial population in each of these treatment systems. In their life processes, these microorganisms use some of the organic matter in order to synthesize new cell material, and they obtain the energy from their synthesis and cell maintenance functions by degrading some of the organic matter to simple compounds. Thus, biological growth involves both cell synthesis and biodegradation processes.
Heterotrophic organisms, which require a complex source of organic carbon for growth, while autotrophic organisms are able to synthesize their organic requirements from inorganic carbon sources such as CO2. Heterotrophic organisms obtain the energy necessary for growth and maintenance functions by breaking down some of the organic food supply. Autotrophic organisms are able to obtain their energy requirements either by oxidizing inorganic ions, in which case they are chemosynthetic, or by utilizing sunlight, the photosynthetic organisms. Aerobic heterotrophic bacteria are organisms responsible for the primary breakdown of organic matter in wastewater treatment. Autotrophic organisms of importance in special cases include the bacteria responsible for nitrification, and algae (and cyanobacteria), which fulfil an important role in contributing oxygen in oxidation ponds.
Anaerobic and facultative heterotrophic bacteria are important in the stabilization of the concentrated organic sludges produced in wastewater treatment and also in the treatment of concentrated organic industrial wastes.
Environmental factors which influence biological growth include temperature, pH, mixing intensity and the presence of toxic agents. Temperature may affect the reaction rate of microorganisms to the extent of doubling for each 10C increase. Different organisms predominate at different temperature ranges, however, so that there is little difficulty in developing a suitable microorganism population in all but the coldest climatic conditions. For optimal biological growth the pH should generally be in the range of 6.5 to 7.5, although growth will occur over the range of pH 4.0 to 9.5. Materials should not be present in toxic concentrations, although it is often possible to develop a microbial population which is acclimatized to quite high concentrations of some toxic materials. Mixing is important, especially in suspended growth systems, to ensure effective contact between the active microorganisms and the organic matter, to prevent accumulation of products of microbial decomposition, and to preserve a uniform environment throughout the volume of the reactor.
Any deficiency in nutrient or environmental factors will inhibit biological growth, and will lead to loss of process efficiency. In any case, process efficiency can be maximized by keeping all conditions of operation as constant as possible.
The biological growth curve The growth of a batch culture of microorganisms utilizing a single growth-limiting nutrient (substrate), such as organic carbon, is illustrated in Figure 5.1, where it is assumed that all other nutrient and environmental requirements as discussed above are satisfied.
Initially, when the food supply is present in excess, the organisms grow at a rate controlled by their inherent metabolic rate, and organism numbers increase logarithmically. This phase is followed by a declining growth phase during which shortage of available food begins to limit the rate of organism growth until at some point, approaching exhaustion of the food supply, the mass of organisms present reaches a maximum. Thereafter, as cells die and are used as a food source by those, which remain, the total cell mass declines, in the process of endogenous respiration.
Figure 5.1 The batch biological growth curve showing typical operating points
One implication of the growth curve in Figure 5.1 is that, in the declining growth phase, the rate of organism growth at any time is a function of the food concentration. Practical biological wastewater treatment processes are continuous rather than batch operations, however, they may be represented, on average, as a single point on the batch growth curve. Each of these operating points is evidently characterized by a particular value of both microorganism concentration and food concentration, known as the food-to-microorganism ratio-F/M.
F/M Ratio Another important characteristic is that not only does the rate of organism growth decline as food supply becomes growth limiting, but the net yield of organism mass per unit mass of substrate utilized also declines. Hence the lower the F/M ratio, the greater will be the proportion of the substrate degraded to supply the energy requirements of the cell, and the lower the rate of accumulation of biological solids in the system.
Sludge age From these observations, it is possible to develop another process parameter which is of value in the design and operation of many biological systems-mean cell residence time, c (also known as the solids retention time or sludge age). This may be defined as the average time a mass of cells remains in a biological treatment system before being withdrawn in the waste solids stream. If the total mass of biological solids in the system is represented by [X]T (in kg), and the daily increase in solids mass, which occurs as a result of growth, by [X]T (in kg/d), then c is given by
c = [X]T days (1)
c can be correlated with a modified form of the F/M ratio to give a relationship which is useful for process design and operation.
In practical systems, the F/M ratio becomes: Mass of BOD added per day (2)
AEROBIC BIOLOGICAL TREATMENT Conventional methods of biological treatment can be classified as either fixed film or suspended growth processes. An outline of the main processes of each of these classifications is given below. Within each classification, the processes are discussed in their approximate order of development.
Fixed film processes Fixed film treatment involves establishing a microbial culture on a fixed medium and having the wastewater contacting the organisms by flowing past.
Land Treatment In land treatment systems (Figure 5.2a), the active biological content includes many forms of soil bacteria, mainly aerobic and facultative forms, and many other higher organisms and vegetation species. Oxygen is transferred from the atmosphere by natural processes. Net growth of biomass in land treatment is usually in the form of vegetation, which must be harvested either as crops or by grazing animals or even by simply mowing the grass.
Land treatment is probably the oldest method of disposing of human and other household wastes and, in rural areas and small communities, is still the accepted method in many countries. For modern cities, with their abundant piped supplies of fresh water and consequently large discharges of wastewaters, enormous areas of land would be required for land treatment. In addition, there is always a risk of heavy stream pollution by storm runoff from land disposal areas, unless provision is made to collect and treat storm-flows prior to discharge to surface water bodies. Land treatment is therefore generally considered to be impracticable for most modern cities.
Because of the difficulty in measuring the active biomass in land treatment, the best available unit of loading is in terms of application in rate per unit of land area(kg/ha.d).
Design and management of land irrigation treatment systems should take account such factors as
a) Climate- rainfall, evaporation, temperature and humidity
b) Soil type, depth and texture
c) Application rates - water, organic matter, plant nutrients (especially nitrogen) and total dissolved solids.
Figure 5.2 Diagrams of the principal biological treatment systems: (a) land treatment (flood irrigation), (b) trickling filter, (c) rotating biological filter, (d) activated sludge, (e) aerated lagoon, (f) waste stabilization (oxidation) pond. Example 1 A flow of 3 ML/d of settled sewage with 250 mg/L BOD and 33 mg/L organic and ammonia nitrogen is to be applied to a land irrigation area. If the limiting nitrogen loading is 300 kg/ha.yr, what is the area of land required, the total annual equivalent depth of effluent applied and the BOD arial loading rate?
Note that 1 mg/L = 1 g/m3 = 1 kg/ML
Total daily nitrogen loading = 3 ML/d x 33 kg/ML = 99 kg/d
Land area required =
= = 120.5 ha
Annual effluent application rate =
= 9087 m3/ha.yr = 0.91 m/yr
Arial BOD loading rate = = 2272 kg/ha.yr
The trickling filter (c 1900) - also called a percolating filter and bacteria bed - consists of a bed of suitable coarse porous media on which grows a biological slime consisting mainly of bacteria, and on which graze various forms of worms and larvae which help to keep the slime active (Fig. 5.2b). Settled sewage is distributed over the surface of the medium and, as it flows down through the bed, the fine suspended and dissolved organic matter is absorbed by the biological film. Oxygen to sustain aerobic biological oxidation is provided by air which circulates through the bed. Clogging of the interstices within the filter bed as the bacteria grow, is usually prevented by portions of the film washed out of the bed by the wastewater flow. This material, which constitutes the net increase in biomass in the system and which would otherwise contribute high BOD and SS concentrations to the effluent, is the removed in final sedimentation tanks (known as ‘humus tanks’) for further treatment prior to disposal.
It is difficult to obtain an equal adequate measure of the active mass of biological solids in a trickling filter. Although the total surface area of the medium gives some indication of the possible areas on which the biomass could grow, both the actual thickness of the biomass and the percentage of it, which is active cannot be practically determined. Therefore, it is customary to take the volume of the medium as the most practical measure of microorganism activity in a trickling filter and so to express organic loading rate in terms of the daily mass of BOD applied per unit volume of filter medium (kg BOD/m3.d). The hydraulic loading rate per unit surface area of filter (m3/m2.d or kL/m2.d) is also important since it affects distribution of the flow over the surfaces of the medium, and hence the quality of contact between the applied organic matter and the active biomass.
Example 2 A settled sewage flow of 3 ML/d with 250 mg/L BOD is applied to 2 trickling filters, each 40 m diameter and 2.5 m deep. Calculate the organic and hydraulic loading rates.
Solution Organic loading rate = = = 0.12 kg BOD/m3.d
Hydraulic loading rate = = = 1.2 m3/m2.d
( = 12 ML/ha.d)
Rotating biological filter
The rotating biological filter (c 1960) or RBF process is a recently-developed method of biological treatment which resembles the trickling filter process in that it uses a biological film grown on solid surfaces, but these are on a large number of closely spaced disc mounted on a shaft which rotates above a shallow basin profiled to the perimeter of the discs (Fig 5.2c). Approximately 40 per cent of the surface of each disc is submerged in the settled sewage flowing through the trough at any time. The shaft slowly rotates, alternately exposing the biological film absorbs organic matter and then, during contact with the atmosphere, it absorbs oxygen, so enabling aerobic oxidation to proceed. The net growth of biomass is washed off the surfaces of the discs and must be removed in final sedimentation tanks before discharge. Organic loading, as noted earlier, in this case is measured in terms of daily mass of BOD applied per unit surface area of disc (gBOD/m2.d).
Example3 A flow of 60000 L/d of settled sewage with 250 mg/L BOD is treated in a two-stage rotating biological filter plant, each stage comprising open shaft 4 m long and bearing 25 x 2 m diameter discs per meter. Calculate the average organic loading rate on the discs.
Solution Total BOD load = (60000 x 250) x 10-3 g/d = 15 kg/d
Total disc area = 2 x 4 x [(2)2 /4]x 2 x 25 = 1257 m2