9 Water and Sewage treatment Plant

Babita Khosla

10.1 Introduction

The development of human societies is heavily dependent upon availability of water with suitable quality and in adequate quantities, for a variety of uses ranging from domestic to industrial supplies. An estimate infers that every year, the wastewater discharges from domestic, industrial and agricultural practices pollute more than two-thirds of total available run-off through rainfall, thereby, what can be called a “man-made water shortages.” (Vigneswaran and Sundaravadivel, 2004) Thus, in spite of seeming abundance, water scarcity is endemic in most parts of the world. It is because of these concerns, the Agenda 21 adopted by the United Nations Conference on Environment and Development, popularly known as the “Earth Summit” of Rio de Janeiro, 1992, identified protection and management of freshwater resources from contamination as one of the priority issue, that has to be urgently dealt with to achieve global environmentally sustainable development.

                                     Figure: 1 Different phases of waste water treatment

The term “Wastewater” properly means any water that is no longer wanted, as no further benefits can be derived out of it. About 99 percent of wastewater is water, and only one percent is solid wastes. The principal objective of wastewater treatment is generally to allow human and industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment.

Water treatment is a process of making water suitable for its application or converting used water into environmentally acceptable water or even drinking water or to its natural state. Thus, water treatment is required before and after depending on the application. The treatment may include mechanical, physical, biological, and chemical methods and is an integrated system  comprising of the conventional series of primary and secondary treatment processes, but also includes tertiary treatment (Figure 1) and individual treatment of certain streams.

All water treatments involve the removal of solids, bacteria, algae, plants, inorganic compounds, and organic compounds. The primary and secondary treatment processes handle most of the nontoxic wastewaters while the water having toxic wastes needs to be pre-treated before adding to this flow.

Table1: Major classes of municipal wastewater contaminants, their significance & origin

A typical waste water treatment plant is the conventional series of primary and secondary treatment processes, but may also includes tertiary treatment. The primary and secondary treatment processes handle most of the nontoxic waste waters; while some pre-treatment of the waste water before being added to this flow is necessary to prevent the damage to the downstream equipment.

10.2 Preliminary treatment: In many waste water treatment plants the preliminary treatment is the part of the primary treatment, which includes only the mechanical processes. The pretreatment of the influent involves one or all the following steps depending upon the kind of the waste water to be treated.

10.2.1 Screening is the first unit operation used at wastewater treatment plants. Screening removes large solid chunks and objects such as rags, paper, plastics, and metals to prevent damage and clogging of downstream equipment, piping, and appurtenances. Some modern wastewater treatment plants use both coarse screen and fine screen filters as a part of the pretreatment process. Coarse screens remove large solids, rags, and debris from wastewater, and the types of coarse screens include mechanically and manually cleaned bar screens, including trash racks.

Fine Screens are typically used to remove material that may create operation and maintenance problems in downstream processes, particularly in systems that lack primary treatment. Typical opening sizes for fine screens are 1.5 to 6 mm (0.06 to 0.25 inches). Very fine screens with openings of 0.2 to 1.5 mm (0.01 to 0.06 inches) placed after coarse or fine screens that can further reduce suspended solids to levels near those achieved by primary clarification.

Source: Design of Municipal Wastewater Treatment Plants, WEF MOP 8, Fourth Edition, 1998

10.2.2 Comminutors and Grinders: The processing of coarse solids using comminutors and grinders reduces their size of coarser particles so that they can be removed during downstream treatment operations, such as primary clarification, where both floating and settle able solids are removed.  Comminuting and grinding devices are installed in the wastewater flow channel to grind and shred material in the size range of 20 mm (0.75 inches). Comminutors consist of a rotating slotted cylinder through which wastewater flow passes. Solids that are too large to pass through the slots are cut by blades as the cylinder rotates, reducing their size until they pass through the slot openings.

Grinders consist of two sets of counter rotating, intermeshing cutters that trap and shear wastewater solids into a consistent typically 6 mm particle size.

10.2.3 Grit Removal: Grit includes sand, gravel or other heavy solid materials that are “heavier” (higher specific gravity) than the organic biodegradable solids in the wastewater. The removal of grit prevents unnecessary abrasion and wear of mechanical equipment, grit deposition in pipelines and channels, as well as the accumulation of grit in anaerobic digesters and aeration basins. Grit removal facilities typically precede primary clarification and follow screening and comminution. Many types of grit removal systems exist, including aerated grit chambers, vortex-type (paddle or jet induced vortex) grit removal systems, detritus tanks (short term sedimentation basins), horizontal flow grit chambers (velocity-controlled channel), and hydrocyclones (cyclonic inertial separation). The collected grit must be removed from the chamber, dewatered, washed, and conveyed to a disposal site. Some smaller plants use manual methods to remove grit, but grit removal is usually accomplished by an automatic method. The four methods of automatic grit removal include inclined screw or tubular conveyors, chain and bucket elevators, clamshell buckets, and pumping.

10.2.4 Flow Equalization: The influent before the actual treatment is subjected to flow equalization in a mixing tank to level out the hour-to-hour variations in flows and concentrations. There are spill pond to retain slugs of concentrated wastes that could interfere with the downstream processes.

10.2.5 Fat and grease removal: In some larger waste water treatment plants, fat and grease are removed by passing the wastewater through a small tank where mechanical skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help the recovery of the fat as froth. Many plants, however, use primary clarifiers with surface skimmers for fat and grease removal.

10.3 Primary Treatment: The equalization tank is followed by neutralization tank where required as streams of different pH partly neutralize each other when mixed. The oils, greases  and suspended solids are removed by floatation, sedimentation, filtration or some-times special equipment is also used to remove grit that gets washed into the waste water.

The primary treatment prepares the wastewater for the next secondary (biological) treatment. This involves the separation of suspended organic matter (or human waste) from the wastewater. This is done by putting the wastewater into large settlement tanks for the solids to sink or settle down to the bottom of the tank. The settled solids are called ‘sludge’. At the bottom of these circular tanks, large scrappers continuously scrape the floor of the tank and push the sludge towards the pump away for further treatment. The rest of the water is then moved to the secondary treatment.

10.4 Secondary Treatment: The secondary treatment is the biological degradation of soluble organic compounds that escapes primary treatment. This process is usually done aerobically in an open, aerated vessel or lagoon where the microorganisms degrade this organic matter, which serve as “food” for them. Microorganisms combine this matter with oxygen from the water to yield the energy they need to thrive and multiply. Unfortunately, this oxygen is also needed by fish and other organisms in the river. So, the heavy organic pollution in the river or water bodies can lead to “dead zones” where no fish can be found and sudden releases of heavy organic loads can lead to dramatic “fish kills”. The water, at this stage, is put into large rectangular tanks. These are called aeration lanes. Air is pumped into the water to encourage bacteria to break down the organic contaminants of sludge that escaped the sludge scrapping process. The biological process is then followed by additional settling tanks (secondary sedimentation) to remove more of the suspended solids and microorganisms called as activated sludge. A fraction of this sludge is recycled in certain processes, but ultimately the excess sludge along with the sediment solids has to be disposed-off. Next, the ‘almost’ treated wastewater is passed through a settlement tank, where, more sludge is formed at the bottom of the tank from the settling of the bacterial action. Again, the sludge is scraped and collected for treatment. The water at this stage is almost free from harmful substances and chemicals. The water is allowed to flow over a wall where it is filtered through a bed of sand to remove any additional particles. The filtered water is then discharged into the water bodies.

About 85% of the suspended solids and BOD can be removed by a well running plant with secondary treatment. Secondary treatment technologies include the basic activated sludge process, the variants of pond and constructed wetland systems, trickling filters, rotating biological contactors and other forms of treatment which use biological activity to break down  organic matter. The existing treatment systems can also be modified so as to broaden the capabilities and performance. One example is the addition of powdered activated carbon (PAC) to the biological treatment process, to adsorb organics that the microorganisms cannot degrade. Another example is to add coagulants at the end of the biological treatment to remove phosphorus and residual suspended solids and nutrients like N and P.

10.5 Disinfection: The disinfection typically with chlorine can be the final step before discharge of the effluent. However, some environmental authorities are concerned that chlorine residuals in the effluent can be a problem in their own right, and have moved away from this process. Disinfection is frequently built into treatment plant design, but not effectively practiced, because of the high cost of chlorine, or the reduced effectiveness of ultraviolet radiation where the water is not sufficiently clear or free of particles.

10.6 Tertiary treatment: Many existing wastewater-treatment systems were built for primary and secondary treatment only, but now tertiary treatment processes are added on beyond secondary treatment in order to remove specific type of residuals. The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality to the desired level by removing more than 99 per cent of all the impurities from wastewater, producing an effluent of almost drinking-water quality. This advanced treatment can be accomplished by a variety of methods such as coagulation sedimentation, filtration, reverse osmosis, and extending secondary biological treatment to further stabilize oxygen-demanding substances or remove nutrients. In various combinations, these processes can achieve any degree of pollution control desired. As wastewater is purified to higher and higher degrees by such advanced treatment processes, the treated effluent can then be reused for urban, landscape, and agricultural irrigation, industrial cooling and processing, recreational uses and water recharge, and even indirect and direct augmentation of drinking water supplies.

The related technology can be very expensive, requiring a high level of technical know-how and well trained treatment plant operators, a steady energy supply, and chemicals and specific equipment which may not be readily available.

10.6.1 Coagulation- Sedimentation

Chemical coagulation sedimentation is used to increase the removal of solids from effluent after primary and secondary treatment. The solids heavier than water settle out of wastewater by gravity in the primary and secondary sedimentation tanks but the lighter particles are made to  settle down with the addition of specific chemicals, like alum Al2(SO4)3, lime (CaO), or ferric salts of iron (Fe3+). With the addition of these chemicals, the smaller particles clump or ‘floc’ together into large masses. The larger masses of particles will settle out in the sedimentation tank reducing their concentration in the final effluent.

10.6.2 Filtration

A variety of filtration methods are available to ensure high quality water. Sand filtration, which consists of simply directing the flow of water through a sand bed, is used to remove residual suspended matter. Filtration over activated carbon results in the removal of: non-biodegradable organic compounds, absorbable organic halogens, toxins, color compounds and dyestuffs, aromatic compounds.

Although there are a number of different methods of filtration are practised, but in tertiary treatment the most mature is pressure driven membrane filtration. This relies on a liquid being forced through a filter membrane with a high surface area and small pore size (0.02- 0.2µm) to remove bacteria, viruses, pathogens, metals, and suspended solids.

10.6.3 Reverse osmosis

In the reverse osmosis process, pressure is used to force effluent through a membrane that retains contaminants on one side and allows the clean water to pass to the other side. Reverse osmosis is actually a type of membrane filtration called microfiltration because it is capable of removing much smaller particles including dissolved solids such as salt.

10.6.4 Nutrient Removal: The nutrients in the form of Nitrogen and Phosphorus present in te treated water are also needed to be removed to prevent “Eutrophication” of the water bodies where the water is discharged.

Nitrogen control: Nitrogen present in the waste water as ammonia can be toxic to aquatic life in certain instances and can be removed by additional biological

treatment beyond the secondary stage. The nitrifying bacteria. are employed for removal of ammonia present in wastewater. These bacteria can biologically convert ammonia to the non-toxic nitrate through a process known as nitrification. The nitrification process is normally sufficient to remove the toxicity associated with ammonia in the effluent but the product formed nitrate is a nutrient and inexcess amounts can contribute to eutrophication in the receiving waters. In such situations where nitrogen must be completely removed from effluent, an additional biological process can be added to the system to convert the nitrate to nitrogen gas. The conversion of nitrate to nitrogen gas is accomplished by denitrifying bacteria in a process known as denitrification.

Effluent with nitrogen in the form of nitrate is placed into a tank devoid of oxygen, where carbon-containing chemicals, such as methanol, are added. In this oxygen-free environment, bacteria use the oxygen attached to the nitrogen in the nitrate form releasing nitrogen gas. Because nitrogen comprises almost 80% of the air in the earth’s atmosphere, the release of nitrogen into the atmosphere does not cause any environmental harm.

     Figure 4: The process of nitrification and denitrification in nutrient removal process(http://nett21.gec.jp/WATER/data/img/wtfig_32-2-2.gif)

Phosphorus control: Like nitrogen, phosphorus is a necessary nutrient for the growth of algae. Phosphorus reduction is often needed to prevent eutrophication before discharging effluent into lakes, reservoirs, and estuaries. Phosphorus can be removed either through chemical or biological processes. In biological process, specific bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched in sludge. They can accumulate large quantities of phosphorus within their cells (up to 20% of their mass) and these biosolids after their separation from the treated water have a high fertilizer value.

Phosphorus removal can also be achieved by chemical precipitation, usually with salts or iron, alum, or lime. This may lead to excessive sludge productions as hydroxides precipitates and the added chemicals can be expensive. Despite this, chemical phosphorus removal requires a significantly smaller equipment footprint than biological removal, is easier to operate, and is often more reliable than biological phosphorus removal.

The existing treatment systems can also be modified so as to improve the performance and broaden the possibilities of waste water treatment.