These are intermediate between land treatment and the other more controlled forms of biological treatment in terms of their requirement for land (Fig. 5.2(f)).
Biological reactions occurring in waste stabilization ponds, however, are more complex than those which occur in the other aerobic treatment processes. In the pond liquid, aerobic heterotrophic bacteria break down the organic matter mainly to CO2. Algae then utilize these breakdown products, notably CO2, together with sunlight energy, to photosynthesize new algal cells, releasing oxygen which helps to sustain the aerobic breakdown process. This process, in which the activity of bacteria and algae is mutually beneficial, is called algal-bacterial symbiosis. In the sludge layer which develops in the bottom of the pond, anaerobic biological processes occur and contribute significantly to the treatment efficiency of most ponds. Excess biomass from the processes taking place above is degraded in the anaerobic processes.
The waste matter applied in the pond influent is thus partly stored on the floor of the pond, partly lost as biodegradation products and partly discharged as biomass, notably algae, in the effluent. Hence, in the degree of effluent quality control they can achieve, ponds are probably somewhat inferior to more complex systems, because of the variable concentration of algae escaping in the effluent.
Estimation of active biomass in stabilization ponds is again impracticable, so that gross arial or volumetric loading rate is often used. Most ponds have much the same depth, however, and many of the important phenomena, such as solar energy entering the pond to promote algal photosynthesis and wind action for mixing pond contents, are related to pond surface area. Therefore, the most widely quoted measure of organic loading rate is the daily mass of BOD applied per unit surface area of pond (either kg BOD/ha.d or g BOD/m2.d), although both depth and detention time are usually quoted as well.
Example 4 Raw sewage at a flow rate of 1.5 ML/d, with 300 mg/L BOD, is treated in a series of 3 ponds, with nominal dimensions 300 m x 100 m, 100 m x 75 m and 100 m x 75 m, and each 1 m deep. Calculate the mean detention time for each pond, the average arial organic loading for the pond system and the organic loading on the leading pond.
Solution For illustrative purposes, slopes of the pond banks are neglected and the calculations are based on the nominal dimensions.
Nominal pond surface areas are A1 = 300 X 100 = 30 000 m2
A3 = A2 = 7 500 m2
and since ponds are 1 m deep, nominal volumes are V1 = 30 000 m3,
V 2 = V 3 = 7 500 m3 Detention time in ponds, t = V /Q (m3/d) and, since Q = 1500m3/d,
t1 = 30 000 = 20 d, t2 = t3 = 5 d
Average arial BOD loading = Total BOD load (kg/d)
Total surface area (ha)
= 300 x 1.5 = 100 kg/ha.d = 10 g.m-2 d-1
Average BOD loading on first pond = 300 x 1.5 = 150 kg/ha.d
This is where most of the treatment takes place. Effectively, the load on the secondary and tertiary ponds is negligible as they act to polish the effluent from a BOD of about 20 down to about 5 mg/L.
Aerated lagoons (Fig 5.2(e)) are intermediate between waste stabilization ponds and the activated sludge process, resembling the former in their general form, their construction in earth and also in their flow sheet, since there is no solids recycle. Aerated lagoons also resemble activated sludge processes, however, in that oxygen is supplied by artificial means rather than by algal photosynthesis. The rate of mixing provided by the aeration system is much more intense in aerated lagoons than the natural mixing which occurs in oxidation ponds, so that a higher solids concentration is kept in suspension, and hence is present in the lagoon effluent. In fact, the mass of biological solids leaving in the effluent each day is equal to the day’s net growth of biomass. Thus, final sedimentation of effluent is necessary to achieve a high degree of effluent quality control.
Organic loading rates for aerated lagoons may be expressed in terms of arial or volumetric loading, as in the case of waste stabilization ponds. Because of the greater control of the mixing intensity in aerated lagoons, however, they allow a more mathematically adequate analysis of the concentration of biological solids in the system in equilibrium with a given applied organic load. It is possible, therefore, to express loading rate as an F/M ratio, in terms of kg BOD/kg MLVSS.d.
Example 5 An aerated lagoon 50 m square at the water surface and 3 m deep ( with banks sloped at 2 horizontal to 1 vertical ) receives a wastewater flow of 2.5 ML/d with 300 mg/L BOD. Calculate the detention time, volumetric organic loading rate and F/M ratio ( assuming that equilibrium volatile SS concentration in the lagoon is 400 mg/L 0.4 kg/m3).
Solution BOD load to lagoon = 300 kg/ML X 2.5 ML/d = 750 kg/d
Lagoon volume (Average side length)2 X depth
= [ 50 - (2 X 3)]2 X 3 = 5808 m3 Therefore, detention time = V = 5808 m3 = 2.32 days
Q 2500 m3/d
Volumetric organic loading = 750 kg BOD/d = 13 kg BOD/ m3.d
F/M ratio = BOD loading
Mass of Volatile SS in lagoon
= ______750 kg BOD/d____
5808 m3 X 0.4kg VSS/ m3 = 0.32 kg BOD/kg VSS.d
Activated sludge process
The activated sludge process is an aerobic, biological oxidation process in which sewage is aerated in the presence of a flocculent, mixed microbial culture known as activated sludge.
Essential elements in the process (Fig 5.2(d)) are the aeration tank, in which the activated sludge and incoming sludge and incoming wastewater are thoroughly mixed (the mixture is known as mixed liquor ) and an abundant supply of dissolved oxygen is provided, a final settling tank for separating the activated sludge from the treated effluent, a return sludge system to recycle settled activated sludge solids back to the influent to the aeration tanks, and a means for withdrawing each day’s net growth of biological solids from the system. The unique feature of activated sludge processes, compared with the other processes discussed above, lies in the fact that there is separate and positive control of the retention time of activated sludge solids and the liquid effluent. Hence, it is amenable to much closer control than are other processes.
The organic loading rate in the activated sludge process is given by the F/M ratio - kg BOD/kg MLVSS.d (kg biomass).
Many modifications of the basic activated sludge process have been developed since it was introduced. Some of these are simple variations aimed at improving the load capacity of the process, others are aimed at varying the quality of effluent produced, while still others are aimed at simplifying operation and maintenance. Some modifications employ flow patterns which differ markedly from those of the basic activated sludge process.
Example 6 Settled sewage at a flow rate of 2.5 ML/d and 300 mg/L BOD is treated in an activated sludge plant which is equipped with two aeration tanks each 20 m long x 6 m wide x 4 m deep, and with a mixed liquor volatile SS (ML VSS) concentration of 2000 mg/L. Calculate the detention time, the volumetric organic loading rate and the F/M ratio.
Solution Detention time, t = = = 0.38 days = 9.2 h
Volumetric organic loading = =
= 0.78 kg BOD/m3.d
F/M ratio = =
= 0.39 kg BOD/kg MLVSS.d
METHODS OF ANAEROBIC BIOLOGICAL TREATMENT Anaerobic processes in wastewater treatment are used mainly for treating the organic sludges removed from the wastewater in primary sedimentation and in final sedimentation following aerobic biological treatment. Simple forms of anaerobic treatment, such as anaerobic ponds and septic tanks, however, are used for treating wastewater (rather than sludge) although, even in these cases, the most intense anaerobic action takes place in the layer of concentrated sludge, which settles to the bottom. Although the poor level of mixing, especially in the simpler processes, makes classification a little difficult, most conventional anaerobic processes are essentially suspended growth systems.
Among the more recent developments in anaerobic treatment methods are fixed film processes such as the upflow anaerobic sludge blanket (UASB) process and the anaerobic filter. In the former case, the sludge organisms arrange themselves into dense settleable balls of a few mm diameter and in the latter, the organisms grow on a gravel bed. The UASB processes are often used in the treatment of high organic food industry waste water. More commonly used in anaerobic treatment are the suspended growth processes, some of which are described below.
Anaerobic ponds (Figure 5.3(a)) are heavily-loaded open ponds, usually 2 to 4 m deep, used as pretreatment ponds in municipal wastewater treatment or in industrial waste treatment. In either case they serve to reduce the organic load applied to a series of facultative or aerobic ponds which are invariably required to further treat the wastewater prior to its discharge into the environment. Odor problems are common with this type of pond, especially during start-up, and in systems having seasonally variable loading patterns. Thus, they should be located well away and down-wind from developed areas.
These are single-story tanks used for treating wastewaters from single households or institutions in areas where piped sewerage is not available. They operate essentially as combined sedimentation and anaerobic digestion tanks. A well-designed tank should provide a chamber in which reasonably quiescent settling is allowed to occur. Solids settle to the bottom of the chamber, forming a sludge layer, while fats and floatables rise to the surface to form a scum layer, which helps to prevent access of oxygen through the liquid surface and also helps to control escape of odors. These three zones (see Figure 5.3(b)) are characteristic of well-operating septic tanks.
Anaerobic digestion takes place mainly in the sludge layer, although some liquefaction of the scum layer also occurs. The rate of digestion is very slow at the low temperatures, which prevail during winter in cooler climates. Sufficient volume must therefore be available to store solids during the period when digestion is poor. In any case, inert and slowly-degradable material accumulates and, unless removed, eventually reduces the liquid volume between sludge and scum layers to the extent that no treatment may occur.
The quality of the effluent is then not better than primary effluent. In common with effluents from other anaerobic processes, septic tanks effluent requires further treatment before it is suitable for discharge to surface waters. Sub-surface disposal through absorption trenches is therefore the most commonly adopted method for on-site disposal.
Imhoff tanks are two-story tanks (Figure 5(c)) which represent an advance on the septic tank in that, although they perform the same functions of sedimentation and anaerobic digestion of sludge, these are done in separate compartments. The incoming wastewater flows through the upper compartment, allowing solids to settle to the bottom of the chamber, which is in the shape of a hopper. At the bottom of the hopper, the solids pass through a baffled outlet into the lower chamber in which anaerobic digestion takes place.
The lower chamber incorporates capacity to store solids during periods of poor digestion or between desludging operations. Gas vents are provided in the top of the outer chamber, while the baffles at the bottom of the settling chamber prevent gases from entering the settling chamber itself. The digestion chamber is also provided with collection hoppers and sludge withdrawal pipes for periodic removal of digested sludge for disposal.
Imhoff tanks, because of the cost of constructing the very deep tanks required, are generally considered economical only for small communities and are no longer commonly used.
Cold digestion is the simplest type of digestion used for stabilizing organic sludges produced in conventional primary and secondary treatment. It is operated without temperature control and hence is only suitable for warm climates where the ground temperature remains well above freezing point throughout the year.
Cold digesters are characterized by stratification into four distinct layers (Figure 5.4(a)):
a Scum zone, where floating materials tend to accumulate
b Supernatant liquor zone, where water released from the digesting solids accumulates
c Active digestion zone, where the anaerobic degradation process takes place
Long retention times are required since the operating temperature is far from the optimum.
Two-stage digestion systems
These have two digestion tanks in series (Figure 5.4(b)). To increase process efficiency, the sludge in the leading, or primary, digester is heated to control temperature, and mixed to ensure effective distribution of the feed sludge and active organisms through the whole of the digester volume. The secondary digester is used mainly to allow separation of digested sludge and supernatant liquor. Digested sludge is drawn from the secondary digester for dewatering and disposal, while the supernatant, is returned to the aerobic biological phase of the plant for further treatment.
Because conditions in two-stage digesters are far better controlled and nearer the optimum, much higher loading rates are possible than is the case with cold digesters. In some cases, methane is collected and used as a fuel to assist in heating the sludge in the primary digester.
High-rate digestion This operates at a much higher loading rate than conventional two-stage digestion, and hence it requires very close control if it is to perform efficiently. Conditions for optimum high-rate digestion are
c Thickening of feed sludge (especially if treating excess activated sludges) to increase solids concentration
d Continuous feeding of raw sludges.
Recirculation of some of the digested sludge solids is sometimes provided. Anaerobic contact process is a further development of the high-rate digestion process which provides for separation and recycling of digested sludge solids. It allows separate control of the hydraulic detention time, t, and the mean cell residence time, c. Because the digested sludge produces gas, some form of degassing system should be provided to prevent settling tank impairment. This process is mainly used for industrial wastewater treatment.