Sewage, and Sullage/Greywater

1.0 Sewage, and Sullage/Greywater

In Nigeria, the major liquid wastes comprise of sullage or grey water and industrial effluents. As the human excreta are separately managed through a large number of practices, the waste water is supposed to be devoid of it. The only exception is when people defecate indiscriminately into drains. Sullage is domestic wastewater other than that which comes from the toilet. It results from food preparation, personal washing, and washing of cooking and eating utensils and clothes. It is also called greywater (to distinguish it from blackwater which describes wastes containing human excreta. Grey water is a Priced Resource. It is being used in urban agriculture. Salad crops which are widely grown on sullage and any available waste water can transmit pathogens. The common favoured crops are: Lettuce, Greens, Carrots, Cabbage, Cucumbers, Tomatoes, and Pepper.

The health hazards posed by sullage are not as serious as those associated with other wastewater containing excrete or septic tank effluent. Counts of faecal indicator bacteria have been reported to be significantly lower in sullage than in septic tank effluent, but the washing of babies' clothes and pappies (diapers) is likely to increase the count substantially. Some data suggest that bacteria grow well in sullage. A substantial danger from pathogens is posed by careless tipping of greywater on the ground. If one particular area is always used, its continual moistness will favour the survival of helminths, such as hookworm, and the breeding of flies and mosquitos. In addition, such an area is more likely to be regarded as a waste dump and so be used for defecation, and this practice will increase the number of parasites. Faeces are not easily seen when the ground is muddy. The main hazard to public health is posed by mosquitos, especially Culex quinquefasciatus, which breed in polluted pond water and may spread bancroftian filariasis. Ponding of sullage is caused by excessive discharge on to the ground, by blockage of surface drains, or by unsatisfactory construction or maintenance of open channels to carry the sullage. Pollution of groundwater by sullage may be of less concern than the pollution threat from other wastewater, because the bacterial and nitrate contents are relatively low.

Sewage or Wastewater originates mainly from domestic, industrial, groundwater, and meteorological sources and these forms of wastewater are commonly referred to as domestic sewage, industrial waste, infiltration, and storm-water drainage, respectively. Domestic sewage results from people's day-to-day activities, such as bathing, body elimination, food preparation, and recreation, averaging about 227 litres (about 60 gallons) per person daily. The quantity and character of industrial wastewater is highly varied, depending on the type of industry, the management of its water usage, and the degree of treatment the wastewater receives before it is discharged. A steel mill, for example, might discharge anywhere from 5700 to 151,000 liters (about 1500 to 40,000 gallons) per ton of steel manufactured. Less water is needed if recycling is practiced. A typical metropolitan area discharges a volume of wastewater equal to about 60 to 80 percent of its total daily water requirements, the rest being used for washing cars and watering lawns, and for manufacturing processes such as food canning and bottling.

2.0 Sewage Disposal – Historical Perspective

The issue of sewage disposal assumed increasing importance in the early 1970s as a result of the general concern expressed in the United States and worldwide about the wider problem of pollution of the human environment, the contamination of the atmosphere, rivers, lakes, oceans, and groundwater by domestic, municipal, agricultural, and industrial waste.

Methods of wastewater disposal dates back to ancient times, and sanitary sewers have been found in the ruins of the prehistoric cities of Crete and the ancient Assyrian cities. Storm- water sewers built by the Romans are still in service today. Although the primary function of these was drainage, the Roman practice of dumping refuse in the streets caused significant quantities of organic matter to be carried along with the rainwater runoff. Toward the end of the Middle Ages, below-ground privy vaults and, later, cesspools were developed. When these containers became full, sanitation workers removed the deposit at the owner's expense. The wastes were used as fertilizer at nearby farms or were dumped into watercourses or onto vacant land.

A few centuries later, there was renewed construction of storm sewers, mostly in the form of open channels or street gutters. At first, disposing of any waste in these sewers was forbidden, but by the 19th century it was recognized that community health could be improved by discharging human waste into the storm sewers for rapid removal. Development of municipal water-supply systems and household plumbing brought about flush toilets and the beginning of modern sewer systems. Despite reservations that sanitary sewer systems wasted resources, posed health hazards, and were expensive, many cities built them; by 1910 there were about 25,000 miles of sewer lines in the United States.

At the beginning of the 20th century, a few cities and industries began to recognize that the discharge of sewage directly into the streams caused health problems, and this led to the construction of sewage-treatment facilities. At about the same time, the septic tank was introduced as a means of treating domestic sewage from individual households both in suburban and rural areas. Because of the abundance of diluting water and the presence of sizable social and economic problems during the first half of the 20th century, few municipalities and industries provided wastewater treatment.

During the 1950s and 1960s, the U.S. government encouraged the prevention of pollution by providing funds for the construction of municipal waste-treatment plants, water-pollution research, and technical training and assistance. New processes were developed to treat sewage, analyze wastewater, and evaluate the effects of pollution on the environment. In spite of these efforts, however, expanding population and industrial and economic growth caused the pollution and health difficulties to increase. In response to the need to make a coordinated effort to protect the environment, the National Environmental Policy Act (NEPA) was signed into law on January 1, 1970. In December of that year, a new independent body, the Environmental Protection Agency (EPA) was created to bring under one roof all of the pollution-control programs related to air, water, and solid wastes. In 1972 the Water Pollution Control Act Amendments expanded the role of the federal government in water pollution control and significantly increased Federal funding for construction of waste-treatment works. US Congress has also created regulatory mechanisms and established uniform effluent standards.

3.0 Sewer Lines

Wastewater is carried from its source to treatment facility pipe systems that are generally classified according to the type of wastewater flowing through them. If the system carries both domestic and storm-water sewage, it is called a combined system, and these usually serve the older sections of urban areas. As the cities expanded and began to provide treatment of sewage, sanitary sewage was separated from storm sewage by a separate pipe network. This arrangement is more efficient because it excludes the voluminous storm sewage from the treatment plant. It permits flexibility in the operation of the plant and prevents pollution caused by combined sewer overflow, which occurs when the sewer is not big enough to transport both household sewage and storm water. Another solution to the overflow problem has been adopted by Chicago, Milwaukee, and other U.S. cities to reduce costs: instead of building a separate household sewer network, large reservoirs, mostly underground, are built to store the combined sewer overflow, which is pumped back into the system when it is no longer overloaded.

Households are usually connected to the sewer mains by clay, cast-iron, or polyvinyl chloride (PVC) pipes 8 to 10 cm (3 to 4 in) in diameter. Larger-diameter sewer mains can be located along the centerline of a street or alley about 1.8 m (about 6 ft) or more below the surface. The smaller pipes are usually made of clay, concrete, or asbestos cement, and the large pipes are generally of unlined or lined reinforced-concrete construction. Unlike the water-supply system, wastewater flows through sewer pipes by gravity rather than by pressure. The pipe must be sloped to permit the wastewater to flow at a velocity of at least 0.46 m per sec (1.5 ft per sec), because at lower velocities the solid material tends to settle in the pipe.

4.0 Composition of Waste Water

The composition of wastewater is analyzed using several physical, chemical, and biological measurements. The most common analyses include the measurements of solids, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), and pH. The solid wastes include dissolved and suspended solids. Dissolved solids are the materials that will pass through a filter paper, and suspended solids are those that do not. The suspended solids are further divided into settleable and nonsettleable solids, depending on how many mg of the solids will settle out of 1 liter of wastewater in 1 hour. All these classes of solids can be divided into volatile or fixed solids, the volatile solids generally being organic materials and the fixed solids being inorganic or mineral matter.

Typical values of solids and BOD5 for domestic wastewater are given in the accompanying table. The organic matter in typical domestic sewage is approximately 50 percent carbohydrates, 40 per cent protein, and 10 per cent fat; the pH can range from 6.5 to 8.0. The composition of industrial waste cannot be readily characterized by a typical range of values because its makeup depends on the type of manufacturing process involved.

The concentration of an industrial waste is usually placed in perspective by stating the number of people, or population equivalent (PE), that would be required to produce the same quantity of waste. PE is most commonly expressed in terms of BOD5. An average value of 0.077 kg (0.17 lb) 5-day, 20°C BOD per person per day is used for determination of the PE. The population equivalent of a slaughterhouse operation, for example, will range from 5 to 25 PE per animal.

The concentration of organic matter is measured by the BOD5 and COD analyses. The BOD5 is the amount of oxygen used over a five-day period by microorganisms as they decompose the organic matter in sewage at a temperature of 20° C (68° F). The COD is the amount of oxygen required to oxidize the organic matter by use of dichromate in an acid solution and to convert it to carbon dioxide and water. The value of COD is always higher than that of BOD5 because many organic substances can be oxidized chemically but cannot oxidize biologically. Commonly, BOD5 is used to test the strength of untreated and treated municipal and biodegradable industrial wastewaters. COD is used to test the strength of wastewater that is either not biodegradable or contains compounds that inhibit activities of microorganisms. The pH analysis is a measure of the acidity of a wastewater.

5.0 Industrial wastes

Industrial wastes may be grouped into 2 broad categories: Process wastes and chemical wastes. The process wastes depend on the nature of the industry, the raw materials processed and nature of the process itself. The industrial wastes may pose serious problems on receiving bodies, e.g. oxygen depletion, emission of noxious gases, fish kills and change in flora and fauna. The common pollution problems include high pollution load in the form of colour, turbidity, odour, heat, suspended solids, dissolved solids, BOD, COD, various inorganic elements, volatile organic compounds, toxic chemicals and others. Similarly, the solid wastes originating from human and other activities and their leachates entering the water and food chains may effect the environment and health. At the moment, a majority of the industries either treat their wastes minimally or do not treat at all.

The options for industrial waste disposal are to discharge into surface waters, coastal waters, land and sewers, if available. Depending on where they are disposed, the effluent quality has to be maintained. The flow rates, weather conditions, and the waste characteristics over a time scale are required before planning a treatment technology.

6.0 Waste Water Management Technologies

There is a need to treat such wastes and bring them back into the cycle of life. Several developments have taken place during the last century to manage the wastes. Earlier days it was thought that “solution to pollution is dilution” and this approach never yielded any positive result. The chemical coagulation using lime, ferric salts and alum did not yield any significant results and on the other hand the rivers have deteriorated to a great extent. The available technologies are: primary treatment, secondary treatment and tertiary treatment. Sedimentation tanks, pH control, trickling or percolating filters, activated sludge process, oxidation pond, oxidation ditches, Root Zone Technology, sludge digestion, aerobic and anaerobic lagoon are some of the technologies which are used in dealing with the liquid wastes. Pretreatment is essential before going for biological treatment.

6.1 Primary Treatment

 The wastewater that enters a treatment plant contains debris that might clog or damage the pumps and machinery. Such materials are removed by screens or vertical bars, and the debris is burned or buried after manual or mechanical removal.
 The wastewater then passes through a comminutor (grinder), where leaves and other organic materials are reduced in size for efficient treatment and removal later.
 Grit Chamber- In the past, long and narrow channel-shaped settling tanks, known as grit chambers, were used to remove inorganic or mineral matter such as sand, silt, gravel, and cinders. These chambers were designed to permit inorganic particles 0.2 mm (0.008 in) or larger to settle at the bottom while the smaller particles and most of the organic solids that remain in suspension pass through. Today, spiral-flow aerated grit chambers with hopper bottoms, or clarifiers with mechanical scrapper arms, are most commonly used. The grit is removed and disposed of as sanitary landfill. Grit accumulation can range from 0.08 to 0.23 cu m (3 to 8 cu ft) per 3.8 million liters (about 1 million gallons) of wastewater.

 Sedimentation - with grit removed, the wastewater passes into a sedimentation tank, in which organic materials settle out and are drawn off for disposal. The process of sedimentation can remove about 20 to 40 percent of the BOD5 and 40 to 60 % of the suspended solids. The rate of sedimentation is increased in some industrial waste-treatment stations by incorporating processes called chemical coagulation and flocculation in the sedimentation tank. Coagulation is the process of adding chemicals such as aluminum sulfate, ferric chloride, or polyelectrolytes to the wastewater; this causes the surface characteristics of the suspended solids to be altered so that they attach to one another and precipitate. Flocculation causes the suspended solids to coalesce. Coagulation and flocculation can remove more than 80 percent of suspended solids.

 Flotation - an alternative to sedimentation that is used in the treatment of some wastewaters is flotation, in which air is forced into the wastewater under pressures of 1.75 to 3.5 kg per sq cm (25 to 50 lb per sq in). The wastewater, supersaturated with air, is then discharged into an open tank; there the rising air bubbles cause the suspended solids to rise to the surface, where they are removed. Flotation can remove more than 75 percent of the suspended solids.

 Digestion - is a microbiological process that converts the chemically complex organic sludge to methane, carbon dioxide, and an inoffensive humus like material. The reactions occur in a closed tank or digester that is anaerobic—that is, devoid of oxygen. The conversion takes place through a series of reactions. First the solid matter is made soluble by enzymes, then the substance is fermented by a group of acid-producing bacteria, reducing it to simple organic acids such as acetic acid. The organic acids are then converted to methane and carbon dioxide by bacteria. Thickened sludge is heated and added as continuously as possible to the digester, where it remains for 10 to 30 days and is decomposed. Digestion reduces organic matter by 45 to 60 percent. Digested sludge is placed on sand beds for air drying. Percolation into the sand and evaporation are the chief processes involved in the dewatering process. Air drying requires dry, relatively warm weather for greatest efficiency, and some plants have a greenhouse like structure to shelter the sand beds. Dried sludge in most cases is used as a soil conditioner; sometimes it is used as a fertilizer because of its 2 percent nitrogen and 1 percent phosphorus content.

6.2 Secondary Treatment

Having removed 40 to 60 percent of the suspended solids and 20 to 40 percent of the BOD5 in primary treatment by physical means, the secondary treatment biologically reduces the organic material that remains in the liquid stream. Usually the microbial processes employed are aerobic—that is, the organisms function in the presence of dissolved oxygen. Secondary treatment actually involves harnessing and accelerating nature's process of waste disposal. Aerobic bacteria in the presence of oxygen convert organic matter to stable forms such as carbon dioxide, water, nitrates, and phosphates, as well as other organic materials. Several alternative processes available in secondary treatment are: trickling filter, activated sludge, and lagoons.

 Trickling Filter- In this process, a waste stream is distributed intermittently over a bed or column of some type of porous medium. A gelatinous film of microorganisms coats the medium and functions as the removal agent. The organic matter in the waste stream is absorbed by the microbial film and converted to carbon dioxide and water. The trickling-filter process, when preceded by sedimentation, can remove about 85 % of the BOD5 entering the plant.

 Activated Sludge- This is an aerobic process in which gelatinous sludge particles are suspended in an aeration tank and supplied with oxygen. The activated-sludge particles, known as floc, are composed of millions of actively growing bacteria and protozoa bound together by a gelatinous slime. Organic matter is absorbed by the floc and converted to aerobic products. The reduction of BOD5 fluctuates between 60 and 85 %. An important companion unit in any plant using activated sludge or a trickling filter is the secondary clarifier, which separates bacteria from the liquid stream before discharge. The inventorDr Gilbert Fowler is given in th ephotograph.

Activated Sludge is modified and various designs are currently available –Conventional, Step Aration, Contact Stabilization, Plug Flow, Completely mix and other types. Oxidation Ditch is another improved design which is more economical and suitable for small scale waste treatment, e.g. poultry, piggery etc. In providing secondary treatment plants, various parameters are to be measured and computed. Some of them are: Hydraulic Retention Rate (HRT), Volumetric loading: BOD5 applied per unit volume of aeration tank, and Organic Loading Rate or F/M ratio – it is the ratio of Kg BOD5 applied per day (representing microbial feed) to Kg MLSS in aeration tank (representing microorganisms). The F/M ratio is the main factor controlling BOD removal. Lower the F/M value, the higher will be the BOD removal

 Septic Tank System - A sewage treatment process commonly used to treat domestic wastes is the septic tank: a concrete, cinder block or metal tank where the solids settle and the floatable materials rise. The partly clarified liquid stream flows from a submerged outlet into subsurface rock-filled trenches through which the wastewater can flow and percolate into the soil where it is oxidized aerobically. The floating matter and settled solids can be held from six months to several years, during which they are decomposed anaerobically.

 Stabilization Pond or Lagoon - Another form of biological treatment is the stabilization pond or lagoon, which requires a large land area and thus is usually located in rural areas. Facultative lagoons, or those that function in mixed conditions, are the most common, being 0.6 to 1.5 m (2 to 5 ft) in depth, with a surface area of several acres. Anaerobic conditions prevail in the bottom region, where the solids are decomposed; the region near the surface is aerobic, allowing the oxidation of dissolved and colloidal organic matter. A reduction in BOD5 of 75 to 85 % can be attained.

 Waste Stabilization Ponds - Perhaps one of the most ancient wastewater treatment methods known to humans are waste stabilization ponds, also known as oxidation ponds or lagoons. They're often found in small rural areas where land is available and cheap. Such ponds tend to be only a meter to a meter and a half deep, but vary in size and depth, and may be three or more meters deep. They utilize natural processes to "treat" waste materials, relying on algae, bacteria, and zooplankton to reduce the organic content of the wastewater. A "healthy" lagoon will appear green in color because of the dense algae population. These lagoons require about one acre for every 200 people served. Mechanically aerated lagoons only need 1/3 to 1/10 the land that unaerated stabilization ponds requires. It's a good idea to have several smaller lagoons in series rather than one big one; normally, a minimum of three "cells" are used. Sludge collects in the bottom of the lagoons, and may have to be removed every five or ten years and disposed of in an approved manner.

6.3 Advanced Wastewater Treatment

If the receiving body of water requires a higher degree of treatment than the secondary process can provide, or if the final effluent is intended for reuse, advanced wastewater treatment is necessary. The term tertiary treatment is often used as a synonym for advanced treatment, but the two methods are not exactly the same. Tertiary, or third-stage, treatment is generally used to remove phosphorus, while advanced treatment might include additional steps to improve effluent quality by removing refractory pollutants. Processes are available to remove more than 99 percent of the suspended solids and BOD5. Dissolved solids are reduced by processes such as reverse osmosis and electrodialysis, ammonia stripping, denitrification, and phosphate precipitation can remove nutrients. If the wastewater is to be reused, disinfection by ozone treatment is considered the most reliable method other than breakpoint chlorination. Application of these and other advanced waste-treatment methods is likely to become widespread in the future in view of new efforts to conserve water through reuse.

 Anaerobic Ponds - Anaerobic ponds are commonly 2-5 m deep and receive such a high organic loading (usually >100g BOD/m3 d equivalent to >3000 kg/ha/d for a depth of 3 m). They contain an organic loading that is very high relative to the amount of oxygen entering the pond, which maintains anaerobic conditions to the pond surface. Anaerobic ponds don’t contain algae, although occasionally a thin film of mainly Chlamydomonas can be seen at the surface. They work extremely well in warm climate (can attain 60-85% BOD removal) and have relatively short retention time (for BOD of up to 300 mg/l, one day is sufficient at temperature >20oC). Anaerobic ponds reduce N, P, K and pathogenic micro-organisms by sludge formation and the release of ammonia into the air. As a complete process, the anaerobic pond serves to separate out solid from dissolved material as solids settle as bottom sludge, dissolve further organic material, break down biodegradable organic material, store undigested material and non-degradable solids as bottom sludge, and allow partially treated effluent to pass out.

These fermentation processes and the activity of anaerobic oxidation throughout the pond remove about 70% of the BOD5 of the effluent. This is a very cost-effective method of reducing BOD5. Normally, a single anaerobic pond in each treatment is sufficient if the strength of the influent wastewater is less than 1000 mg/l BOD5. BOD removals and Nitrogen Changes in Anaerobic Ponds loaded at 250 g BOD5/m3d. In anaerobic ponds organic nitrogen is hydrolyzed to ammonia, so ammonia concentrations in anaerobic pond effluents are generally higher than in the raw wastewater (unless the time of travel in the sewer is so long that all the urea has been converted before reaching the WSP). Volatilization of ammonia seems to be the only likely nitrogen removal mechanism occurring to some extent in anaerobic ponds.

 Fluidized Bed Technique – Sridhar and his colleagues designed a modified version of this and used this technology to treat sewage in 1974 which is currently being used by Ademoroti and his colleagues at University of Benin in treating sewage and other industrial wastes. Sewage treatment using plastic beads under aeration in a reactor proved the importance of attached micro-organisms particularly ciliate protozoa- Vorticella sp. in the biological treatment.

 Root Zone Technology- Yet another technology widely studied in Nigeria (Sridhar and his colleagues 2005) , certain aquatic plants (Pistia and Water Hyacinth), and terrestrial plants (Phragmites, Canna lily, Ipomea and others) can also be effectively used for the treatment of small volumes of municipal wastewater, particularly where construction of a sewage collection system to an adjacent wastewater treatment facility would be prohibitively expensive.

 Wetlands as Wastewater Treatment Systems - Natural wetlands have been and continue to be used as wastewater disposal sites. Their ability to act as a buffer, absorbing some of the substances released, has afforded some protection to the rivers, lakes, and estuaries to which they are linked. However, a large proportion of wetlands have been destroyed for construction or agricultural purposes. Apart from the fact that wetlands are vital habitats for a large variety of phylogenetic species, the loss of their role as natural purification systems may be the most critical aspect of this destruction. In wastewater treatment, wetlands are utilized mostly as a chemical sink. In natural ecosystems, wetlands perform other important roles in the transformation of inputs and sources of chemicals for the neighbouring systems. The biogeochemical conditions and factors that define these roles of the particular wetland need to be determined in order to optimize the contaminant trapping aspect.

 Adsorption, Ultrafiltration and other Technologies - In the wastewater treatment, the technology option depends on the goal. One should find out which element or substance is to be reduced or eliminated from the effluents. BOD, SS, Volatile organics, some specific chemicals such as P, N, S, or heavy metals or toxic chemicals have to be addressed. On certain situations adsorbents such as activated charcoal, clays (kaolin or bentonite) may be very useful. Ultrafiltration is becoming more acceptable and the cost of the technology is coming down as more and more innovations are being adopted.

In addition to these treatment methods, sludge has to be disposed of hygienically. Sludge digestion is a preferred method and biogas and fertilizer are the byproducts.

7.0 Conclusions

Hitherto, in Nigeria the emphasis in waste management has been limited to solid wastes. Proper treatment is not practiced in many industries in the country. The regulatory bodies on the other hand have been very sympathetic and have been giving ‘holidays’ for industry for not implementing any treatment facilities. Almost after 13 years of its creation, FEPA (Ministry of Environment where appropriate) have to go into action on enforcing effective treatment and adhering to the stipulated standards.
• There is need to strengthen Federal, State and selected private laboratories to cope with the needs and demand of waste characterization;
• At the moment many unqualified ‘touts’ have entered into the field as ‘Environmental Consultants’; there is need to screen and weed them out;
• Quality assurance, inter-laboratory standardization and periodic training and retraining of all ‘Environmental Practitioners’ should be made mandatory;
• Environment is multidisciplinary and in tackling the problems, a team effort is more beneficial; where necessary many should join hands in solving the problems;
• A National ‘Green Peace’ Group should be formed and encouraged to monitor the industries, communities and regulatory bodies in keeping the environment clean and habitable; and
• There is need to take stock of the availability of technologies and the expertise within the country through proper screening and draft them to address the environmental challenges.

Selected References

Ademoroti, C. M. A. and Sridhar, M. K. C. (1979), Fluidized bed technique in physicochemical treatment
Effluent and Water Treatment Journal, U.K., 19: 291-297
Hammer, M. J. (1986), Water and Wastewater Technology, SI Version, Second Edition, John Wiley & Sons, New
York, pp 1-536
Peavy, H. S., Rowe, D. R. and Tchobanoglous, G. (1985), Environmental Engineering, McGraw-Hill International
Editions, Civil Engineering Series, New York, pp. 1-677
Sridhar, M. K. C. and Oyemade, O. (1987), Health risks at sewage treatment plants in Ibadan, Nigeria
Journal of Institution of Water and Environmental Management, U.K., 1: 129-135
Sridhar, M. K. C. (1991), Textile mill wastes in Nigeria--Problems and treatment options, Proceedings of a Special
Symposium on Emerging Technologies for Hazardous Waste Management, Edited by D. Willaim Tedder, , Atlanta, Georgia, Industrial and Engineering Division, American Chemical Society, pp.13 in a Volume of pp 1-431
Sridhar, M. K. C. (1995), Sullage / Waste Water in Nigeria: Problems and Solutions for Utilization for Gardening, A
report submitted to UNICEF, Lagos, Nigeria, July, pp.1-86