22-12-2010, 01:58 PM
BY: P.D.NITYA
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Need For Low Energy- Intensive Methods For Treating Low Strength - Large Volume Wastewater (like sewage, industrial washwater, etc.)
1.1 Definition of Low Strength Wastewater
Typical composition of untreated domestic wastewater (influent) was defined using concentration levels as weak, medium, and strong. Weak, medium, and strong levels of BOD5 (mg/L) were identified as 110, 220, and 400, concentrations of TSS (mg/L) were identified as 100, 220, and 350, and concentrations of FOG(Fat, Oil, Greases) (mg/L) as 50, 100, and 150 respectively (Tchobanoglous and Burton, 1991).
The character of household wastewater is identified using average and maximum levels. Average BOD5 levels were identified as 200 mg/L with a maximum of 400 mg/L, average TSS levels of 200 mg/L with a maximum of 400 mg/L, and average Grease levels of 50 mg/L with a maximum of 150 mg/L (Laak, 1986). Most researchers apply a range to quantify concentrations of BOD5, TSS, and FOG for wastewater influent. The most commonly used range is 100-300 mg/L BOD5 and 100-350 mg/L TSS. FOG ranges typically are identified as 50-150 mg/L with 150 mg/L being identified as “strong”.
However, statistically as the research progressed, new parameters determining the range of BOD5 and COD also changed, though with not much variation. The Table 1.1 shows the parameter which classifies the strength of wastewater based on its BOD5 and COD concentrations.
Fresh wastewater is a grey turbid liquid that has an earthy but inoffensive odour. It contains large floating and suspended solids (such as faeces, rags, plastic containers, maize cobs), smaller suspended solids (such as partially disintegrated faeces, paper, vegetable peel) and very small solids in colloidal (i.e. non-settleable) suspension, as well as pollutants in true solution. It is objectionable in appearance and hazardous in content, mainly because of the number of disease-causing (‘pathogenic’) organisms it contains. In warm climates wastewater can soon lose its content of dissolved oxygen and so become ‘stale’ or ‘septic’. Septic wastewater has an offensive odour, usually of hydrogen sulphide. (Mara ,2004)
1.2 Sources Of Low Strength Wastewater
They are mostly domestically generated and sometimes constitute of washwater from industrial treatments. Its main sources are:
• Cesspit leakage;
• Septic tank discharge;
• Sewage treatment plant discharge;
• Washing water (personal, clothes, floors, dishes, etc.), also known as greywater or sullage;
• Rainfall collected on roofs, yards, hard-standings, etc. (generally clean with traces of oils and fuel);
• Groundwater infiltrated into sewage;
• Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, etc.);
• Urban rainfall runoff from roads, car parks, roofs, sidewalks, or pavements (contains oils, animal faeces, litter, fuel or rubber residues, metals from vehicle exhausts, etc.);
• Direct ingress of river water (high volumes of micro-biota);
• Blackwater (surface water contaminated by sewage);
• Industrial waste
• industrial site drainage (silt, sand, alkali, oil, chemical residues);
o Industrial cooling waters (biocides, heat, slimes, silt);
o Industrial process waters;
o extreme pH waste (from acid/alkali manufacturing, metal plating);
o agricultural drainage, direct and diffuse.
1.3 Existing Methods Of Wastewater Treatment And Their Limitations
Few of the methods and techniques used for treatment of low strength domestic and industrial wastewater includes the following
Fixed Film Systems
Fixed film systems grow microorganisms on substrates such as rocks, sand or plastic. The wastewater is spread over the substrate, allowing the wastewater to flow past the film of microorganisms fixed to the substrate. As organic matter and nutrients are absorbed from the wastewater, the film of microorganisms grows and thickens. Trickling filters, rotating biological contactors, and sand filters are examples of fixed film systems. (Hamid,2003).
BIOFILTERS
Biofilters (also called trickling filters, percolating filters and bacteria beds) are an old process for the secondary treatment of domestic wastewater dating from the beginning of the 20th century (Institution of Water and Environmental Management, 1988), Biofilters comprise a 2–3 m deep bed of 50–100 mm rock. The settled wastewater – that is, the effluent from either a primary sedimentation tank, or in developing countries more appropriately an anaerobic pond (Broome et al, 2003) – is distributed mechanically over the rock medium and it percolates down through the medium to be collected in an underdrain system at the base of the bed. A microbial film develops on the surface of the rock and the bacteria in this ‘biofilm’ oxidize the wastewater organics (i.e. remove the BOD) as the settled wastewater trickles down through the bed.The biofilm develops in thickness as the bacteria in it grow on the settled wastewater. Eventually it becomes too thick and some is sheared off (often described as ‘sloughed off’) by the wastewater flow. The solids (sometimes called ‘humus solids’ or simply ‘humus’) have to be removed in a secondary sedimentation tank (a ‘humus tank’) or in a sedimentation or maturation pond, so that the final effluent has a sufficiently low suspended solids concentration.
LIMITATION
Bacteria are not the only organisms to live in biofilters. There is a whole complex ecosystem of protozoa, metazoa, worms – and fly larvae. Unless controlled, swarms of newly emergent adult ‘filter flies’ (such as Psychoda and Sylvicola) and chironomid midges can make life unpleasant for nearby residents (Surrey Advertiser, 1998). Fly control is best achieved by covering the rock medium with high-density polyethylene netting (Palmhive Technical Textiles, 2002), which overall demands quite a maintenance involving too many resources. (Mara, 2004)
Suspended Film Systems
Suspended film systems stir and suspend microorganisms in wastewater. As the microorganisms absorb organic matter and nutrients from the wastewater they grow in size and number. After the microorganisms have been suspended in the wastewater for several hours, they are settled out as sludge. Some of the sludge is pumped back into the incoming wastewater to provide "seed" microorganisms. The remainder is wasted and sent on to a sludge treatment process. Activated sludge, extended aeration, oxidation ditch, and sequential batch reactor systems are all examples of suspended film systems. (Hamid,2003).
OXIDATION DITCH
The oxidation ditch was developed in The Netherlands by Arthur (1983). Oxidation ditches are a direct modification of conventional activated sludge (Baars, 1962; Barnes et al, 1983; Environmental Protection Agency, 2000). Their essential operational features are that they receive raw wastewater (after preliminary treatment) and provide longer retention times: the hydraulic retention time is commonly 0.5–1.5 days and that for the solids 20–30 days. The latter, achieved by recycling >95% of the activated sludge, ensures minimal excess sludge production and a high degree of mineralization in the small amount of excess sludge that is produced. Sludge handling and treatment is almost negligible since the small amounts of waste sludge can be readily dewatered without odour on drying beds. The other major difference is in reactor shape: the oxidation ditch is a long continuous channel, usually oval in plan and 2–3 m deep. The ditch liquor is aerated by several aerators, which impart a velocity to the ditch contents of 0.3–0.4 m/s to keep the activated sludge in suspension. The ditch effluent is discharged into a secondary sedimentation tank to permit solids separation and sludge return and to produce a settled effluent with low BOD and SS. Removals consistently >95 per cent are obtained for both BOD and SS. Oxygen is required at a rate of ~1.5 g O2/g BOD applied. Such a rate of supply includes an allowance for the endogenous respiration of the sludge and maintains aerobic conditions along the entire length of the ditch. (Mara, 2004)
SEQUENCING BATCH REACTOR
The Sequencing Batch Reactor (SBR) is an activated sludge process designed to operate under non-steady state conditions. An SBR operates in a true batch mode with aeration and sludge settlement both occurring in the same tank. The major differences between SBR and conventional continuous-flow, activated sludge system is that the SBR tank carries out the functions of equalization aeration and sedimentation in a time sequence rather than in the conventional space sequence of continuous-flow systems. In addition, the SBR system can be designed with the ability to treat a wide range of influent volumes whereas the continuous system is based upon a fixed influent flow rate. Thus, there is a degree of flexibility associated with working in a time rather than in a space sequence. Sequencing batch reactors operate by a cycle of periods consisting of fill, react, settle, decant, and idle. The duration, oxygen concentration, and mixing in these periods could be altered according to the needs of the particular treatment plant. Appropriate aeration and decanting is essential for the correct operations of these plants. The aerator should make the oxygen readily available to the microorganisms. The decanter should avoid the intake of floating matter from the tank. (Abreu and Estrada,2005)
LIMITATION
The aerated systems require a steady supply of oxygen to maintain the activated sludge suspended so as to provide contact for microorganisms to reduce the organic matter. They demand a long retention time for treatment often producing bad odour.
Lagoon Systems
Lagoon systems are shallow basins which hold the waste-water for several months to allow for the natural degradation of sewage. These systems take advantage of natural aeration and microorganisms in the wastewater to renovate sewage. (Hamid, 2003)
AERATED LAGOONS
Aerated lagoons are activated sludge units operated without sludge return. Historically they were developed from waste stabilization ponds in the northern US, where mechanical aeration was used to supplement the algal oxygen supply in winter. It was found, however, that soon after the aerators were put into operation the algae disappeared and the microbial community quickly came to resemble that of activated sludge. Aerated lagoons, especially those operating at short retention times in warm climates, are designed as completely mixed non-return activated sludge units. Floating aerators are used to supply the necessary oxygen and mixing power. Aerated lagoons can treat either raw wastewater (after preliminary treatment), or settled wastewater, (Kouraa et al, 2002) and Daqahla, Egypt (El Sharkawi et al, 1995). BOD removals above 90 per cent are achieved at short retention times (2–6 days); retention times less than 2 days are not recommended as they are too short to permit the development of a healthy flocculent sludge (even so the activated sludge concentration is only 200–400 mg/l, in contrast to the 2000–6000 mg/l found in conventional systems and oxidation ditches). (Mara, 2004)
Biological treatment processes that utilise wind-aerated lagoons have proved popular for small communities because of their negligible sludge production. Horan, Salih et al reports an intensive 12-month study designed to both monitor the lagoon performance and establish the key design parameters that the final effluent reports for biochemical oxygen demand and total suspended solids, which averaged 9 and 28 mg/L, respectively. The lagoons showed an accelerated growth of algae during the summer months, but this did not adversely affect the final effluent suspended solids. The lagoons also achieved a faecal coliform removal of around 3.4 log and an average effluent ammonia of 7.6 mg/L. However, the ammonia removal was seasonal with a better performance in the summer months, which probably reflects take-up by the growing algal population over this period. No sludge was wasted from the lagoon over the monitoring period yet it accumulated only sparingly in the lagoons, mainly around the inlet of the primary lagoon. The capital costs make this more expensive than other, similar options, but plant operating costs are significantly reduced. (Horan, Salih et al. 2006)
Aerated submerged bio-film (ASBF) pilot plant has been developed to optimize an inexpensive method of enhanced wastewater treatment. Choi, Johnson et al research explores the possibility of enhancing the performance of shallow wastewater treatment lagoons through the addition of specially designed structures. These structures are designed to encourage the growth of a nitrifying bacterial bio-film on a submerged surface. These structures also force the direct contact of rising air bubbles against the submerged bio-film. This direct gas-phase contact should increase the oxygen transfer rate into the bio-film, as well as increase the micro-climate mixing of water, nutrients, and waste products into and out of the bio-film. This research investigated the efficiency of dissolved organic matter and ammonia nitrogen removals at cold temperatures. As it was anticipated, nitrification activity was highest during periods when the flow rate was lower, but it seemed to decline during times when the flow rate was highest. And ammonia nitrogen removal rates were more sensitive than dissolved organic matter removal rates when flow rates exceeded 2.2 L/min. (Choi, Johnson et al. 2010)
LIMITATION
In common with all activated sludge systems, aerated lagoons are not particularly effective in removing faecal bacteria. Extra external power has to be supplied to ensure thorough mixing, though nutrient removal efficiency is quite high.
Final Treatment
Final treatment focuses on removal of disease-causing organisms from wastewater. Treated wastewater can be disinfected by adding chlorine or by using ultraviolet light. High levels of chlorine may be harmful to aquatic life in receiving streams. Treatment systems often add a chlorine-neutralizing chemical to the treated wastewater before stream discharge.
Advanced Treatment
Advanced treatment is necessary in some treatment systems to remove nutrients from wastewater. Chemicals are sometimes added during the treatment process to help settle out or strip out phosphorus or nitrogen. Some examples of nutrient removal systems include coagulant addition for phosphorus removal and air stripping for ammonia removal.
Sludge is generated through the sewage treatment process. Primary sludge, material that settles out during primary treatment, often have a strong odor and require treatment prior to disposal. Secondary sludge is the extra microorganisms from the biological treatment processes. The goals of sludge treatment are to stabilize the sludge and reduce odors, remove some of the water and reduce volume, decompose some of the organic matter and reduce volume, kill disease causing organisms and disinfect the sludge. Untreated sludge are about 97 percent water. Settling the sludge and decanting off the separated liquid removes some of the water and reduces the sludge volume. Settling can result in sludge with about 96 to 92 percent water. More water can be removed from sludge by using sand drying beds, vacuum filters, filter presses, and centrifuges resulting in sludge with between 80 to 50 percent water. This dried sludge is called a sludge cake. Aerobic and anaerobic digestions are used to decompose organic matter to reduce volume. Digestion also stabilizes the sludge to reduce odours. Caustic chemicals can be added to sludge or it may be heat treated to kill disease-causing organisms. Following treatment, liquid and cake sludge are usually spread on fields, returning organic matter and nutrients to the soil. (Wastewater Source Guide KISR/NSTIC-2003 6)
1.4 NEED FOR ENERGY-INTENSIVE OPTIONS
All the fore stated methods for treatment of low strength wastewater requires high capital investment, maintenance and since the recovery of organic waste is less, most of the times, the domestic, sewage waste is discharged without treatment, in the main stream of rivers.
The overall importance for treating low strength wastewater can be strengthened if anaerobic process treatments be effectively utilized to recover biogas formed and minimize sludge production. Also the large scale generation of domestic low strength wastewater per day calls on the need to develop low energy consuming methods which must enhance the quality of effluents, doesn’t need much maintenance and make use of by-products generated. The large quantity of aerobic sludge generated by treatment processes, mainly waste activated sludge, involves large capital and operating costs. Thus, there is a considerable need to find energy effective and energy producing wastewater treatment alternatives.
Wastewater treatment for re-use in agriculture and aquaculture can be subjected to classical benefit–cost analysis using discounted cash-flow techniques to show if the present value of future additional crop yields is more than the present value of wastewater treatment. However, wastewater treatment prior to discharge to inland or coastal waters is less easy to analyse. Central government, with its national perspective, must set national environmental and environmental health priorities. It can enforce these by lending money only for wastewater treatment projects that lie within these priorities. Generally, and ideally, priority projects should be dealt with on the basis of river basin catchment areas, as this is the best method of integrated water resources management, with central government deciding which river basin is (or which river basins are) to be protected first, what level of protection is needed now and how this can be developed to progressively higher levels of protection in the future for wastewaters are simply too valuable to waste. (Mara, 2004)
1.5 THE PROMISE OF UASB
UASB (Upflow Anaerobic Sludge Blanket) systems cost half as much as conventional aerobic treatment systems, and use 30 to 40 per cent less energy. Today, more than 3,000 industrial and domestic treatment systems worldwide - about eight in 10 of the world's anaerobic wastewater treatment systems - use the technology. Such reactors can be found in sugar refineries, breweries, paper mills and other industries. Anaerobic sludge stabilization is one of the most successful and promising treatment processes, exhibiting several significant advantages over aerobic stabilization; reduction of pathogenic organisms and sludge production; saving an air supply, prevention of nuisance- odour conditions after digestion (Prayoon, 1992)
Advantages of UASB process
• Low investment cost.
• Low land requirements.
• Low energy costs, just transport influent to the plant.
• Production of valuable by product- Biogas.
• Very high loading rate can be applied, including for low strength domestic wastewater.
• Short retention time.
• Preservation of anaerobic sludge in the reactor for many months without loosing much of its activity is possible.
• No need of support medium as required in other high rate anaerobic process.
• Low production of stabilized excess sludge, which can be easily dewatered.
• Acceptable effluent quality with high COD removal efficiency (65-90%).
• Simple reactor construction.
• Nutrient requirement is low.
