The activated sludge process surely is the most widely used biological process for the treatment of municipal and industrial wastewaters. Normally, the activated sludge process is strictly aerobic, although anoxi1 variations are coming into use for denitrification,. In simple terms, the activated sludge process consists of a reactor called the aeration tank, a settling tank, solids
recycle from the settle to the aeration tank, and a sludge wasting line. The aeration tank is a suspended-growth reactor
containing) microbial aggregates, or flocs, of microorganisms termed the activated sludge. The microorganisms consume and oxidize input organic electron donors collectively called the BOD. The activated sludge is maintained in suspension in the reactor through mixing by aeration or other mechanical means. When the slurry of treated wastewater and microbial flocs pass to the settling tank, the flocs are removed from the treated wastewater by settling and returned to the aeration tank or wasted to control the solids retention time (SRT). The clear effluent is discharged to the environment or sent for further treatment. Capturing the flocs in the settler an recycling them back to the reactor are the keys to the activated sludge process, because they lead to a high concentration of microorganisms in the reactor. Thus, the sludge is “activated” in the sense that it builds u to a much higher concentration than could be achieved without the settler and recycle. The high biomass concentration allows the liquid detention time to be small, generally measured in hours, which makes the process much more cost effective. Wasting the sludge through the separate sludge-wasting line makes the solids retention time (SRT ) separate from and much larger than the hydraulic detention time.
The basic principle of aerobic treatment is that the waste-water is brought into contact with a mixed microbial population of aerobic organisms and oxygen. Soluble, suspended and
colloidal biodegradable materials that contribute to the BOD are then metabolized:
Aerobic microbes + BOD +O2
New cells (biomass) + CO2 + residual BOD + H2O During the process, part of the biodegraded material is converted into CO2 (mineralization) and a proportion becomes new biomass (assimilation). Under ‘starvation’ conditions, some of the microbial biomass (intracellular storage compounds) may also be metabolized; this is referred to as endogenous respiration.
A major problem associated with aerobic treatment is the disposal of excess biomass produced during the degradation of the pollutants. Approximately 30-70% of the biodegraded carbon is transformed into new cells, and the remainder is converted to CO2, the specific values being process dependent. Although the efficiency of the systems relies on the production of new active cells, this simultaneously produces a new form of pollution, the excess waste biomass, which must be safely disposed of. Therefore, aerobic treatment can be classed as only 30-70% efficient, depending upon the specific process. The fermentor, with a depth of 100m, is sunk in the ground to reduce noise, odour and land usage, and has significantly lower biomass yields. Neverertheless, the most commonly used aerobic processes are still the conventional activated sludge processes and trickle filters described below.
An outline of conventional wastewater treatment. An activated sludge plant
Homogeneous activated sludge Processes
The activated sludge process was originally developed in 1914 by Arnold and Locket. The basic principles of the process are that the waste-water is brought into contact with a mixed microbial population, in the form of a flocculated suspension, within a continuously aerated and agitated tank. These processes are routinely used to treat domestic wastewater and industrial waste-waters that have usually undergone primary treatment. Typically, primary treated wastewater entering the system contains150-200mg/L TSS, 150-200mg/L BOD, 20-40mg/L ammoniacal nitrogen and 6-10 mg/L phosphorus. However,
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these values vary depending on the location and the nature of the wastes deposited into the system treatment
This is a two-stage process, involving biological treatment and secondary settlement. Biological treatment is performed in an aerated basin containing a diverse range of flocculated microorganisms, the mixed liquor suspended solids (MLSS), that biodegrade the polluting material present. As the microorganisms grow in the aeration basin they clump (flocculate) together to form stable flocs of activated sludge. The formation of stable flocs of 2-3 mm diameter is essential for the efficient operation of the plant with respect to BOD removal and rapid settlement in the secondary sedimentation stage.
The microorganisms present include a range of bacteria, e.g. carbon oxidizers, filamentous carbon oxidizers, nitrifies, denitrifies, etc., along with fungi, protozoan, withers, nematodes and algae. Despite being widely used, the microbiology and community structure of activated sludge processes is not well characterized. However, bacteria such as species of Acinetobacter and Zoogloea ramigera are considered to playa key role in floc formation by the synthesis and secretion of polysaccharide gels. Protozoa act as bacterial scavengers, ensuring low turbidity in the final treated effluent. Some 200 protozoan species have been isolated, but the sludge recycle most
important are the ciliated forms, e.g. Vorticella apercu/aria. Overall, activated sludge must contain a microflora capable of producing all enzyme systems required for the biodegradation of both soluble and insoluble pollutants. These
microorganisms should form flocs with good absorbing properties that are stable and settle rapidly.
Secondary settlement occurs after one hydraulic retention time, when the treated effluent from the aeration basin passes into a secondary settlement tank. This is similar in design to primary sedimentation, i.e. SLR, 15-30 m3/m2/day; SLR, 50-100kg/ m2/day; HRT, 2-4h. Here the flocculated microorganisms rapidly settle to form a secondary sludge, normally containing 1- 3% (w/v) total solids, and a clarified supernatant. Often the supernatant is suitable for final disposal, but where necessary it can be subjected to a tertiary treatment to remove inorganic nutrients.
Depending on the design sludge loading rate (SLR), a proportion of the settled flocculated MLSS (secondary sludge) is returned to the aeration basin to maintain the required operating MLSS (microbial biomass) concentration. This allows a high concentration of biomass to be maintained in the aeration basin independent of the growth rate of the microorganisms, thus preventing microbial wash-out. These systems are comparable to stirred tank bioreactors with biomass recycle.
Numerous designs and configurations of activated sludge plants exist, but they vary in only four key aspects: sludge loading rate (see below), MLSS concentration (kg/m3), configuration and method of oxygen supply. The main parameters that have to be taken into account when designing activated sludge systems are:
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Hydraulic loading rate (HLR, kg BOD/m3/day) raw BOD (kg/m3) x flow rate (m3/day) Aeration tank volume (m3)•
Hydraulic retention time (HRT, h)•
Reactor volume (m3)•
Flow (m3/h)•
Sludge loading rate (SLR, kg BOD/kg MLSS\day)•
Raw BOD (kg/m3) x flow rate (m3/day)•
MLSS (kg/ m3) x aeration tank volume (m3)The operating SLR affects the level of treatment achieved and is basically the food to biomass ratio, which is the mass (kg) of BOD provided per kilogram of biomass (MLSS) per day. Therefore, as a rule and up to a limit, the more food (BOD) that is added to each kilogram of MLSS, the faster the microorganisms grow (see Chapter 2, Microbial growth). However, for maximum purification (percentage BOD removal) the food to biomass ratio should be low. This maintains the cells in a partially starved state, thereby ‘encouraged’ to actively metabolize any biodegradable pollutants present, to produce a low residual BOD (i.e. the substrate is limiting). Conversely, with increasing food to biomass ratios, the food availability increases, allowing higher microbial growth rates and greater biomass yields. Also, as substrate is no longer limiting its residual concentration in the final effluent increases.
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What are the different modes of operation of activated sludge plants?There are three main modes of operation for activated sludge plants: conventional, extended aeration and high rate treatment. The major difference is the operating SLR .However, percentage BOD removal, HRT, biomass yield and sludge age (residence time) vary depending on the nature of the waste being treated. This is particularly true for industrial waste-waters whose concentration and composition vary considerably. As a rule, the more concentrated the influent BO D, the longer the required operating HRT to achieve the necessary degree of treatment and the lower the sludge age.
Conventional processing is used for complete treatment of waste-waters such as domestic wastewater. Here the lower the SLR operated, the greater the level of purification obtained. Over 95% removal of BOD can often be achieved at the lower end of the SLR range (0.25kg BOD/m3/day), falling to 85% removal at the higher end of the range (0.5 kg BOD/m3/day). Extended aeration operates at a lower SLR than conventional plants and achieves approximately the same degree of
purification, but the operating HRT is significantly longer. First impressions suggest that this system has no advantages over conventional treatment, particularly as this system is often more costly to construct and operate. Higher costs are due to increased HRTs that require a reactor with a greater volume and,
consequently, more energy for aeration. Nevertheless, the main advantage of this system is the significantly reduced biomass yield (0.2-0.3 kg biomass per kilogram of BOD removed). This is approximately 50% of that found in conventional plants and substantially reduces the costs of its disposal. The reduced biomass yield is a function of the lower SLR, which maintains
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the cells in starvation conditions, so that a proportion of cells respire endogenously.
High rate treatment is mostly used for the partial processing of strong industrial waste-waters and is designed to remove only 60-80% of BOD. Consequently, the treated waste-waters normally remain too polluted for direct disposal into the environment. The high food to biomass ratio (kg BOD: kg MLSS) favours the faster-growing- microorganisms and results in an increased biomass yield (0.5-0.7kg biomass per kilogram of BOD). However, the high SLR produces short HRTs. Generally, for any given waste-water, the lower the SLR, the larger the aeration basin volume required. This results in longer operating HRTs, reduced biomass yields and improved percentage BOD removal. The opposite occurs as the SLR increases. A smaller aeration basin volume is required, which gives reduced HRTs, increased biomass yields and lower BOD removal rates.
Any specific SLR value chosen is therefore a function of the degree of treatment required, land availability, running costs and the cost of disposal of the excess sludge generated.
Dissolved oxygen in activated sludge plants
The operating dissolved oxygen (DO) level is a function of the value chosen in the design requirements. If the objective is full nitrification, a dissolved oxygen concentration of at least 2 mg/ L is necessary. However, where only carbon oxidation and denitrification is required, a lower DO will suffice. The DO concentration required within the aeration basin can be determined by either mathematically modeling the system to predict oxygen demand at different times of the day, or by use of feedback mechanisms incorporating oxygen electrodes. Oxygen requirements may be supplied by mechanical aerators installed in the aeration basin, bubble diffusers (spargers), or a combination of the two.
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All right. So this is how we treat the liquid part. And what to do about the sludge and the solid waste generated?Disposal of sludge has to be done to meet the local regulations laid down by the governing bodies.
Methods routinely used for the disposal of final sludges and other solid wastes are as follows:
1. Landfilling: which is also used for other agricultural, industrial and urban wastes. However, there are potential pollution problems as materials can leach into adjacent water courses when unsuitable or ill-prepared sites are used. Also, it is becoming increasingly expensive due to a lack of suitable landfill sites. Nevertheless, landfilling has potential as a means of methane production. This may be provided that problems associated with the establishment of suitable microbial populations can be overcome, possibly by inoculation with appropriate methanogens.
2. Incineration: is routinely used for solids and well dewatered sludges with solids contents in excess of 30% (w/v). For sludges, the system operates with limited energy input due to their high calorific value, leading to self combustion.
3. Biologically stabilized dewatered sludge: may be used as a low-cost fertilizer and soil conditioner on agricultural land, often incorporated with composted solid organic agricultural and household wastes. This mode of disposal is becoming increasingly popular, but the regulations regarding disposal are very stringent, particularly regarding nitrogen,
phosphorus and heavy metal concentrations, and pathogen content.
The fine colloidal particles present in the wastewater water ,are removed in the form of gelatinous precipitate. This precipitate is named floc. The precipitation is carried out by adding some chemical coagulants like alum, ferric chloride, ferric sulphate, etc. to the wastewater water. The heavy metals present in the wastewater water are treated before the disposal of the.
wastewater water. Thus the wastewater water becomes more or less pure, which is fit for agricultural purpose.
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·Well, now what is the next type of treatment? It is called trickling filters.The basic principle of aerobic trickle filters is that a microbial population is allowed to develop as a biofilm on an inert support material within a biological reactor. Polluted waste- water is continuously sprayed over the surface of the support material and percolates (trickles) through the filter bed, where it is biodegraded by the microbial population. Aeration is achieved by exploiting the difference in temperature between the inside and outside of the reactor, resulting in a counter current of air. High microbial activity within the reactor causes a rise in temperature, and the warm air rises and allows fresh air to enter at the bottom of the reactor.
As treatment proceeds, the biofilm grows and increases in depth until a critical thickness is achieved, at which point oxygen becomes limiting at the surface of the support material. This results in the biomass falling off, called sloughing, after which the biofilm starts to redevelop.
Microbial populations vary considerably depending on the position within the filter. At the top, a range of
microorganisms develops, including bacteria, fungi, protozoan and algae; along with microorganisms, especially insects and their larvae. Below the surface, carbon-oxidizing
microorganisms predominate, whereas nitrifies are mostly found at the bottom of the filter. Overall, a highly complex food chain is created within these filters.
The three most important features of the packing material (inert support material) are as follows.
1 The specific surface area to volume ratio for biological attachment: the larger the surface area, the greater the biomass concentration per unit volume of the reactor and therefore the faster the rate of biodegradation.
2 The voidage volume: a high void space is required to prevent clogging and short-circuiting of the waste-water as it passes through the filter bed. Also, a high voidage volume aids oxygen transfer.
3 The density of the support material: the more dense the support material, the stronger the construction has to be to support and contain the total weight.
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·What are the different types of trickling filters? Trickle filters can be designed to operate under two modes of operation. Low rate filters almost invariably have stone or some other dense mineral medium with a low surface area and high density, whereas high rate filters use plastic media with high voidage and high surface areas. As these systems are non-homogeneous and have complex ecology, it is impossible to quantify the total biomass concentration attached to the inert support material. When designing such filters it is not practical to use the sludge loading rate (kilogram of BOD per kilogram of MLSS per day) as in the activated sludge process, because the biomass concentration is unknown. Therefore, it is normal to use the organic loading rate (OLR, kilogram of BOD/m3/day), which is the mass (kg) of BOD added to each m3 working volume of the reactor per day. This does not take into account the microbial biomass concentration within the bioreactor. Low rate filters as used in wastewater works are usually designed to produce effluents of high quality. They employ mineral support material (e.g. slag and granite), which develop a mature biological film within 424 weeks. These mineral low rate filters are normally circular or rectangular. Their depth is often restricted to 1.5-2.5 m, due to the dense nature of the support material and the associatedconstruction costs. Most circular filters do not have diameters greater than 40m and rectangular filters are less than 75 m long and 45 m wide. Mineral support media usually have surface areas in the region of 80-110m2/m3 and a voidage of 45-55%. This relatively low void age can result in filter blockages, known as ponding. When operating such filters it is important that the biomass should not be allowed to dry out as it affects their overall efficiency. Therefore, recycling of clarified supernatant from secondary sedimentation is often required.
Low rate filters operate with an OLR of 0.06-0.12kg BOD/ m3/day, a wetting rate of 0.5-4.0m3/m2/day, and an HRT in the region of 20-60 min. They remove 90-95% of the BOD, resulting in high-quality effluent.
High rate trickle filters are often used for treating concentrated industrial waste-waters, acting as a ‘roughing’ process rather than a complete treatment, comparable with the high rate activated sludge process. They remove 50-80% of the BOD and their OLR is about la-fold higher than with low rate filters. To overcome problems associated with low rate filters (dense support material, low surface area for attachment, potential ponding problems, and limited depth), plastic support materials have been developed. These plastic materials are chemically stable, but are gradually degraded by light. Their low density reduces associated civil engineering costs and permits filter depths of 6-9 m, which minimize land requirements. In operation, a high voidage volume, normally greater then 95%, reduces ponding. The very large surface area for microbial attachment, normally in the range 100-300 m2/m3, also results in high biomass concentrations. This allows greater OLRs to be used, while maintaining good levels of BOD removal. However, the large surface area of the support material necessitates higher
wetting rates, in order to keep the biomass moist. Recycling of clarified supernatant from the secondary sedimentation tank satisfies this requirement.
Overall, high rate filters are defined as those where the operating OLRs are in excess of 0.6 kg BOD/m3/day, but this may reach as high as 10 kg BOD/m3/day. However, the higher the operating OLR, the lower the degree of
purification attained. For example, depending on the nature of the waste, at an OLR of 1 kg BOD/m3/day, BOD removal efficiencies of 80-90% can be expected, falling to approximately 50% with OLRs of 3-6kg BOD/m3/day.
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Ok, now what happens after the secondary process? After the secondary process, the tertiary treatment is carried out. Tertiary TreatmentTertiary treatment is any additional treatment process designed to achieve higher standards of water quality. Disinfection systems such as ultra-violet or micro filtration remove any remaining viruses after secondary treatment and can remove up to 99.9% of faecal bacteria. Other tertiary treatments, called nutrient stripping, concentrate on nitrogen and phosphorus removal.
Several techniques are available to remove dissolved salts from wastewater effluent, but all are quite expensive.
Ultra-Violet (UV)
UV light systems appear to present no threat to the marine environment, since this treatment is non-additive (i.e. does not involve use of chemicals). Most water companies have a number of UV tertiary treatment schemes included within the current improvement programme.