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close this bookUrban Wastewater Projects - A Layperson's Guide (EEA, 1998, 124 p.)
close this folderChapter 2. An Introduction to Urban and Rural Wastewater Management
View the document(introduction...)
View the document2.1 Chapter Content
View the document2.2 The Nature of Urban Wastewater
View the document2.3 Sewer Systems
View the document2.4 Industrial Effluents
View the document2.5 Rural Area Sewage
View the document2.6 Wastewater Treatment
View the document2.7 Effluent Disposal and Beneficial Use
View the document2.8 Sludge - Treatment, Disposal and Beneficial Use

(introduction...)


Figure


Fig. 2 No caption

2.1 Chapter Content

The purpose of this chapter is to provide an introduction to the subject of urban wastewater and the issues it raises with respect to its management in the best interests of the public and the environment in which it lives.

A list of questions commonly asked by those embarking upon the planning of an urban wastewater project for the first time has been compiled. Many of the topics are dealt with in greater detail in later chapters.

This chapter has been divided into the following sections:

2.2

The Nature of Urban Wastewater



2.3

Sewer Systems



2.4

Industrial Effluents



2.5

Rural Area Sewage



2.6

Wastewater Treatment



2.7

Effluent Disposal and Beneficial Use



2.8

Sludge - Treatment, Disposal and Beneficial Use

2.2 The Nature of Urban Wastewater

2.2.1 What is urban wastewater?

The main component of urban wastewater is sewage from domestic dwellings, offices and other commercial premises. Liquid effluents from industrial processes and service industries such as laundries are also commonly discharged into urban sewers together with domestic sewage. Older sewer systems were also designed to receive rainwater, drainage from streets, roofs and other paved and impermeable areas. Although this practice continues today, it is increasingly common to provide a separate network for rainwater.

2.2.2 What is domestic sewage?

The liquid waste produced by domestic activities has two main components:

· “Grey water”; Water that has been used for baths, showers, in wash-basins and the washing of clothes and floors, and

· “Black water”; Water and waste from toilets and kitchen sinks. In turn, toilet waste in areas not served by sewer systems may be termed “night soil”, as it is separately stored and carted away from the house.

Normally both of these components, black water and grey water, are combined and discharged into a single drainage system and together are referred to as “domestic sewage” or simply “sewage”.

2.2.3 Why differentiate between “grey water” and “black water”?

Grey water contains very little solid material and under the right circumstances can be considered suitable for recycling.

If plumbing systems permit the separation of the two components, grey water can be used for watering gardens in times of drought. However, the adverse influence of increasing amount of detergents in grey water on gardening should be kept in mind. In very exceptional circumstances, i.e. in chronically water-short areas, grey water may be treated at the place of origin and reused for toilet flushing.

It should be emphasised that the recycling of grey water imposes not only significant additional costs on housing construction but also its treatment is subject to many problems. Reuse and recycling of grey water is not yet a common practice.

2.3 Sewer Systems

2.3.1 Why have we developed a waterborne system for carrying away our waste?

In early urban settlements, waste was dealt with at individual dwellings. The quantity of water used was considerably less than today. Water would need to be drawn from wells or the nearest watercourse and carried or hauled to the dwelling. Only rarely was water piped to centres of population and even more rarely to individual houses. Water used for personal hygiene, the washing of floors and for cooking soaked away into the ground. Human excreta was at best stored and this “nightsoil”, as it is sometimes called, was either carted away to a tip or watercourse or, as was common in China from early times, used as a fertiliser in agriculture.

As towns grew in size, it became an onerous task to cart away nightsoil from an increasing number of houses. The Romans designed and constructed drains beneath the streets to carry the water and nightsoil mixture away by gravity to ditches and watercourses.

However, with the passing of the Roman Empire, it appears that its drainage techniques and practices fell into disuse. Although a few towns could be said to have systematised their water supply and wastewater infrastructure throughout the ages, in general, most towns and cities grew unplanned and lacked any form of system to carry away waste. Where a system was provided, it generally consisted of open ditches along the centre of streets into which all manner of domestic waste was thrown. These ditches were occasionally cleaned by the municipal authorities but more often than not were left to fill and fester until a storm carried away the accumulated mess. Open, natural watercourses collected the waste from the streets and when the flow in them was sufficient, the wastes were carried away from the city. Crowded cities stank from these practices and periodic bouts of dysentery and bubonic plague, which decimated medieval urban populations from time to time, were a natural consequence of such unhygienic practices.

2.3.2 So what stimulated the invention and construction of our modern system of sewers to carry away the wastewater that we produce?

With the advent of the industrial revolution in Europe in the late 1700s, there was a considerable migration of the rural population to the towns that grew rapidly in size. Local wells and handpumps could no longer cope with the demand for water and, in addition, human waste seeped into the aquifers and contaminated the water withdrawn from wells for local use. The invention of steam-driven pumps enabled clean water in considerable quantities to be brought to the cities from sources remote from centres of population. Increasingly, a clean and abundant water supply was made available to cities and towns, initially at public standpipes in the streets and gradually, through a system of pipes, direct into houses and factories.


Fig 2.1 Small diameter sewer under construction


Fig. 2.2 Effluent-producing factory

From the early 1800s, it became necessary to construct a system of drains to carry away such large quantities of water if the streets of towns and cities were not to become continually awash. Industrialised production of the water-closet at about this time made its installation in houses more common and further increased the pressure for sewer systems to be constructed. Quite naturally, drains constructed to carry away water used for laundering and for personal hygiene were also used to carry away water closet waste.

Although existing open watercourses and storm drains had been used for some time to carry away waste, these were now culverted, roofed over, and additions to the system were constructed as closed culverts from the outset. In time, sewer systems were designed to comprehensively serve all development and where a gravity system could not cope alone, pumping stations were constructed.

At first, wastewater was carried away to the edge of developments and discharged untreated into watercourses, lakes and the sea. As wastewater quantities increased and the link between sewage, disease and hygiene became firmly established in the middle of the 19th century, conditions in rivers, lakes and coastal waters became aesthetically and hygienically unacceptable. In order to reduce the polluting potential of the wastewater, treatment plants were constructed from about this time. Initially sewage was either subjected to settlement or conveyed to farmland on the outskirts of towns and cities. So common and important was this practice that the public still often refer to modern, sophisticated treatment plants as “sewage farms”.

Treatment techniques have now been developed to the point that, if necessary, under extreme circumstances, wastewater can be treated and reused as drinking water. However, under most circumstances, it is neither environmentally necessary nor economic to treat to such a high standard.

2.3.3 What does an urban wastewater system consist of?

An urban wastewater system is composed of a sewer system, a wastewater treatment works and an effluent discharge pipe. Sewer systems are described in Chapter 5 and wastewater treatment works in Chapters 6 and 7.

2.4 Industrial Effluents

2.4.1 What are industrial effluents?

Industrial effluents are the liquid wastes from industrial processes.

In some cases, industrial effluents are similar in their constituents to domestic sewage, e.g. those from food processing, soft drinks manufacture or laundries, although they are often stronger and produced in considerable quantity. In others, they may contain material that would be toxic or corrosive to discharge into a watercourse or sewer untreated, e.g. those from many chemicals processing plants, refineries, urban gasworks, electroplating factories and metal-pickling and paint workshops. Some wastes may be akin to domestic sewage but are extremely polluting due to high concentrations of organic material such as blood, oils and grease, e.g. effluents from dairies, slaughterhouses, breweries and distilleries.

2.4.1.1 When should the discharge of industrial effluents into sewers be permitted?

With the exception of the effluent from largest industrial sites such as petroleum refineries, it is generally accepted that, subject to the control of their quality and quantity, most industrial effluents should be discharged into the public sewer system for treatment at the municipal works. There are a number of reasons for this:

· Many industrial effluents are readily treated by the same processes normally installed at a municipal wastewater treatment works and some are more easily treated when mixed with domestic sewage than on their own.

· If required to treat the effluents on site, before discharge into a sewer or a watercourse, the plant will often be poorly operated and maintained, as effluent treatment is rarely considered an integral part of the industrial process. This results in the need for considerable surveillance by the regulatory authorities. Hence the expertise of the municipal plants can be exploited in ways that may not exist on the factory.

· All industrial effluent treatment plants produce sludges and many produce screened material. These must either be treated at the site of the industry or carried away for treatment. This can cause considerable odour and disruption of traffic and, depending upon the type of industry, can be potentially hazardous.

· In some cases, effluent treatment adds considerably to the cost of industrial production and it is not good for the competitiveness of industry, and thus the local economy, if higher than necessary treatment costs are imposed on industry. The larger the treatment works, the lower the cost of treating a cubic metre of wastewater. Therefore, to combine industrial effluents with domestic sewage, will result in a lower treatment cost. The cost of conveying the industrial effluent and of treating it can be calculated and should be charged back to the industry.

· In several countries the discharge of treated/untreated industrial wastewater will be allowed only after thorough assessment, as it can contain constituents that may adversely affect sludge quality, thus affecting its reuse potential in agriculture.

However, before an industry is permitted to discharge its processed effluents into the public sewer, the public authority should agree on the conditions for discharge. These will include a limit on the maximum hourly and daily effluent flow, limits on physical, chemical and bacteriological content and the charges to be levied for accepting the effluent into the sewers and treating it. The conditions should be expressed in a legally binding licence.

It is now common in the EU to practise the precautionary approach. That is to say, an industry is required to demonstrate that effluents whether, untreated or treated will not adversely affect the public wastewater system, failing which, permission should not be granted for their discharge into the sewers. In all cases it will be beneficial to the industry, the community and the environment to encourage processes and practices that produce a minimum of waste.

Many sources of information exist on the quality limits to be placed on industrial effluents before they can be accepted into the public sewer system and on methods for charging for this service in a fair and equitable manner.

Before an industry is permitted to discharge its effluents into the public sewer system, it should be required to demonstrate that they do not contain substances which, in the sewers, either alone or mixed with sewage or other effluents, could:

· produce toxic or explosive atmospheres;

· be corrosive to the fabric of the sewers or machinery in contact with the sewage;

· have a detrimental effect on the sewers and the treatment processes at the municipal wastewater works;

· have a detrimental effect on the use and disposal of the final effluent and sludge by-products;

· suddenly or progressively block the sewers, e.g. excessive amounts of oil or grease;

· cause flooding by causing pump failures.

In addition, the effluents should be neither excessively hot or cold. It may be that an industry will need to install a plant to pre-treat their effluents in order to make them acceptable for discharge into the sewers.

2.4.1.2 When should an industry be refused permission to discharge its effluents into the public sewers?

If an industry cannot comply with the quality or quantity limitations that the authorities wish to set, then it must make arrangements for treatment at its own site and for the conveyance of the effluent to a watercourse.

However, there are other concerns that may influence a decision to give or refuse permission for an industry to discharge its effluents into the public sewers.

A town or city normally has a long term existence. Industry is not so permanent. Due to market uncertainties, an industry cannot be sure of its future existence beyond the short term, often less than 2 or 3 years. Sewer systems are designed and built to last for 50 years or more, the structures in Wastewater treatment plants 30 years and machinery 10 to 15 years.

If additional sewer capacity or treatment units have to be provided in order to accommodate an effluent from industry, the municipal service provider will need to be certain of recovering the significant investment made, from the industry concerned. It is difficult to be precise as to when additional sewer or treatment capacity will be needed. However, consideration will need to be given to this when flows and pollution loads from a single industry exceed 5% of the municipal wastewater flow and some additional capacity will almost certainly be needed when this figure exceeds 15%.

In the event that additional capacity would need to be built, there are a number of options:

· the industry concerned can pay directly for the additional capacity, or

· it can make financial provision to guarantee repayment in the event of closure or change of needs due to a reduction in their manufacturing processes or production capacity.

If the industry can satisfy neither of these, discharge of its effluents may be refused and it will need to treat the effluents on its own site. However, it can be complicated by many economic factors, for example the additional development of an area may be positively stimulated by various authorities in a particular area through subsidies or grants or reduced contributions.

2.5 Rural Area Sewage

2.5.1 How is sewage from rural areas treated?

In rural areas, domestic sewage is normally dealt with on individual premises in cesspools and septic tanks.

Cess-pools are tanks without an outlet that are used solely to store the sewage. The stored waste must be frequently removed as the tank is filled. Cess-pools are expensive to operate due to the frequent necessity to empty them. Cess-pools are installed only when the ground is impermeable or the water table rises to near or above ground level for all or part of the year.


Fig. 2.3 Factory-built treatment plant for small, rural developments

Septic tanks are small underground tanks interred in the ground, away from the houses that they serve and which act as small treatment plants with a low efficiency. Solids settle to the floor of the tank and oils and grease rise to the surface of the tank contents. A clarified effluent is preferably dispersed into the ground through a soakaway system. Less acceptably, the effluent may be discharged to a ditch or watercourse but its quality is such that this may give rise to odour problems or pollute the recipient. Periodically, sludge is sucked from the septic tank by a purpose-built road tanker and conveyed to a plant for further treatment and safe disposal.

For larger premises in rural areas, such as hotels and restaurants, a small treatment plant, often factory-built, will be installed. These plants use the same basic treatment processes as municipal wastewater plants. However, arrangements are normally made to convey the sludges that they produce to a municipal plant for treatment and safe disposal.

2.5.2 When is the transition made from individual on-site treatment to the construction of a sewer system to transmit sewage to communal treatment?

A sewer system and communal treatment plant are constructed when it is either cheaper to do so than construct and operate individual on-site facilities, or when the ground on which the development is situated is insufficiently permeable to absorb the discharges from septic tanks. Under these circumstances, unsanitary conditions and odour nuisance may result from this practice or the quality of the groundwater may be reduced to an unacceptable level.

2.5.3 Do rural wastewater treatment plants for small populations create particular problems?

Ideally, a wastewater treatment plant would be served by a short sewer system, receive a constant flow rate of wastewater and be of a size to justify a full compliment of technical and support staff working on-site. Sewer systems and small treatment plants serving scattered rural populations and villages do not generally satisfy these criteria and so the problems created must be taken into account when planning their design and operation, viz.:

· long lengths of sewer and pumping mains serving small populations have long retention times and wastewater can become septic, creating offensive odours and difficulties in treatment;

· the smaller the population served, the more variable is the wastewater flow rate and the pollution load arriving at the treatment works throughout the day and treatment units must be designed to take this into account;

· it is often difficult to allocate operational staff exclusively to a small treatment works; if there are a number of such works in an area, the formation of a mobile operations and maintenance team might be justified, otherwise regular visits should be made by the staff from a larger works, suitably trained in the operational problems of small units.

2.6 Wastewater Treatment

2.6.1 Why is there a need to treat wastewater?

Briefly, the treatment of wastewater is practised to avoid otherwise unacceptable conditions e.g.:

· risks to public health

· pollution of natural bodies of water into which effluents are discharged -watercourses, lakes and the sea - to the point where they damage aquatic plant and animal life or prevent their normal economic, social or recreational use through contamination or deoxygenation

· pollution of the general environment by creating offensive odours or sights and the contamination of groundwater.

In addition, the provision of wastewater treatment is good social practice, there being a general public aversion to finding sanitary waste in water bodies of environmental importance.

The European Community’s Urban Wastewater Treatment (UWWT) Directive requires that all Member States pass legislation which ensures that developments having a population above a given figure to treat their wastewater before discharge into watercourses, lakes or the sea. However, the degree of treatment to be provided and the date by which such treatment must be operational, depends partly upon the size of the population served and partly upon the sensitivity of the body of water into which the effluent is discharged. This is further detailed in Chapter 3, Section 3.3.

Wastewater treatment is described in greater detail in Chapters 6 and 7.


Fig. 2.4 Medium-sized wastewater treatment plant

2.6.2 What are the detrimental effects of discharging untreated wastewater into a recipient and what are the benefits of wastewater treatment?

Wastewater Constituent

Detrimental Effects

Benefit of Wastewater
Treatment for Community




Large solid material -paper, rags, plastic bags, condoms, etc.

Unsightly - accumulate as litter on banks of rivers, lakes and beaches

River banks, lakes and their surroundings and beaches are rendered more pleasant and safer environments for work and for recreation


Can constitute a risk to health on contact

Improved economy where based on recreation and/or tourism




Organic matter - food waste, faecal matter and some industrial effluent

Oxygen levels in receiving waters are reduced by bacteria and higher orders of aquatic life consuming the organic matter - fish and other organisms die and eventually disgusting odours are produced -similar to rotten eggs and rotten cabbage

Livelihoods dependent upon fishing are protected as is fishing for sport



More pleasant environment for living, working and recreation



Improved economy where based on recreation and/or tourism




Oils and greases

Unsightly and potentially damaging and harmful scum formed on water surfaces Impermeable film on water surface reducing potential for, water to absorb oxygen from atmosphere

Improved oxygen absorption into the water body from atmosphere assisting aquatic life to survive



More pleasant environment for living, working and recreation






Improved economy where based on recreation and/or tourism




Nutrients - nitrogen, phosphorus and trace materials

Act as fertiliser and stimulates growth of algae, seaweed and other aquatic plants choking watercourses and littering banks of rivers and lakes and beaches with rotting material, eventually becoming organic waste

Improved and safer conditions for shellfish cultivation and other aquatic organisms


Can stimulate toxic algal blooms which accumulate in shellfish and can infect humans who consume them

More pleasant environment for living, working and recreation



Improved economy where based on recreation and/or tourism




Disease-causing bacteria and viruses - e.g. cholera, typhoid and salmonella

Contamination of water resources used for drinking or irrigation of crops eaten raw by humans or animals

Improved public health Improved and safer conditions for shellfish cultivation and other aquatic organisms


Contamination of water used for shellfish cultivation

Improved economy where based on recreation and/or tourism


Contamination of water used for water contact sports





Toxic substances - generally originating from industrial effluents

Dependent upon toxicity and concentrations in receiving water can

Improved conditions for aquatic life



Improved public health



- destroy or damage aquatic life




- accumulate in flesh of fish, shellfish and creatures which feed upon them and eventually affect humans consuming them


2.7 Effluent Disposal and Beneficial Use

2.7.1 What are the options for effluent disposal?

Treated effluent from a wastewater treatment plant is normally discharged into the nearest water body capable of accepting it without detrimentally affecting it. This can be a drainage ditch, a river, stream or torrent, a lake or the sea.


Fig. 2.5 Agricultural re-use of treated effluent

In some cases, to ensure adequate and effective dilution of the effluent in the receiving water, an underwater pipeline must be constructed, equipped at its discharge end with a diffuser system. This is particularly so, when treated effluent is discharged into the sea and acceptable bacterial levels are required at the shoreline and inshore waters without resorting to disinfection. The issue of mixing zones is important and the environmental/economic balance of several small discharges versus one large discharge needs careful assessment.

In addition to providing dilution, a long offshore outfall provides time for natural bacterial die-off to occur before the considerably diluted effluent reaches the shoreline. Techniques exist for predicting the dilution, dispersion and die-off that will be achieved.

2.7.2 Under what circumstances is it worthwhile to reuse treated effluent for beneficial use?

Recycling treated effluent for further use is unlikely to be either necessary or worthwhile where natural water resources are sufficient to satisfy all normal demands placed upon them by an area, e.g. satisfactorily serving the needs of the population, commerce and industry, the public services, landscaping and agriculture.

However, if water resources are either periodically or continually unable to satisfy water demands in the area, the recycling of treated effluent for beneficial use should be considered.

It is generally possible to reuse effluent for use as irrigation water in agriculture, landscaping and forestry, although, unless it has been either disinfected or stored for some time, care should be taken in its use on crops eaten raw, or where spray irrigation techniques are used.

Treated sewage effluent can be used for secondary industrial purposes such as cooling and quenching without treatment other than disinfection and dosing with algaecides.

Some water deficient areas have insisted on dwellings and public buildings being provided with dual plumbing systems for the water supply, one for toilet flushing and the other for all other water uses. It is possible to use treated effluent in the toilet flushing system as long as suitable precautions have been taken against cross-connection with the potable supply, e.g. colour coding of pipes and “labelling” of the effluent, and the effluent has been disinfected and dosed with algaecide.

If other uses are under consideration, such as in industrial processes or for drinking purposes, it will be necessary to subject the effluent to considerable further treatment as for any other primary quality potable supply. As effluent is generally of lower quality than natural water sources, this can be very expensive and has been practised in very few locations.

2.8 Sludge - Treatment, Disposal and Beneficial Use

2.8.1 What is wastewater sludge?

These solid residues are separated from the watery effluent from the process either by physical screening and filtration or through sedimentation. Because these processes take place in a watery environment and it is difficult to remove all of the water, the residues are not dry and, in fact, generally contain a high proportion of water prior to their further treatment. This mixture of solids and water is termed “sludge”.

Sludge is, in effect, a concentration of the polluting material in the wastewater, either in its original form or transformed by the treatment processes. It has a high organic content and, if not treated, rapidly putrefies and gives off objectionable odours. It is therefore subjected to a series of treatments in order to stabilise it and to remove sufficient water from it to enable it to be disposed of without its causing nuisance or polluting the environment.

Depending upon the degree of stabilisation and dewatering required, facilities for the treatment of sludge can cost between 30% and 50% of the cost of the treatment plant.

Sludge is more viscous than sewage and its treatment often poses more problems in operation than wastewater treatment. However, it is essential to appreciate that:

· no treatment plant can operate without producing sludge at some time and normally it is produced in significant quantities every day

· no treatment plant should be built without ensuring that there is a secure means of disposing of the sludge

· every wastewater treatment plant must be provided with sludge treatment processing facilities capable of adequately preparing the sludge for the disposal option chosen.

Sewage sludge utilisation is regulated by the EU Directive 86/278/EEC. The Urban Wastewater Directive 91/271/EEC prohibits marine disposal of sludge after 1998. However it should be noted that several countries have much stricter national regulations.


Fig. 2.6 Liquid sludge from wastewater treatment process is stabilised and dewatered prior to disposal or beneficial use

2.8.2 What are the disposal options for wastewater sludge?

There are not many options for the disposal of sludge. The two most common are disposal into a sanitary landfill and the beneficial use of sludge in agriculture, horticulture and forestry.

Disposal into a sanitary landfill can either be as dewatered sludge or, following incineration, as ash. When disposed into landfill, an especially prepared area, rendered impermeable, should be used, if it is necessary to protect the groundwater below the site from pollution. Surface water runoff and drainage from the sludge from this area will need to be subjected to treatment.

The use of sludge in agriculture is the preferable alternative. However it should be appreciated that the sludge can contain disease-causing bacteria, sometimes in a cyst form resistant to disinfection, as well as organisms that can harm certain crops. It may well be necessary to carry out some form of pasteurisation of the sludge or long-term storage to avoid harmful effects. There are a number of processes that convert the sludge into a form of product that is easier and safer to handle in agriculture.

In addition, wastewater sludge contains concentrations of metals where these originate from industrial discharges. It should be possible to reduce their concentration through controls on the industries concerned, in order to remain within the acceptable limits for metals in soils.

2.8.3 Are there any other methods of disposal or beneficial uses of sludge?

In the past, disposal of sludge at sea was commonly practised by urban developments at or near the coast. This practice is prohibited from 1998 by EU Directive, 91/271/EEC.

Considerable work has been carried out on beneficial use options. Particularly noteworthy is the recent research that has been done in Japan. The use of sludge has been considered for fuel, in ceramic production and in the formation of building panels. It has even been used to produce decorative brooches, tie-pins and table mats! At present, these uses are still to be considered experimental and are generally more costly than conventional disposal or agricultural use. They can only be considered by the very largest of wastewater plants serving populations in excess of a million.

Where sufficient landfill space is not available and when the sludge is not suitable for use in agriculture, incineration will have to be considered. Ash resulting from incineration, which will be far smaller in volume compared to the original sludge, must also be properly disposed of according to the same restrictions as sludge.

2.8.4 What happens to screened material as well as grit and sand removed in the preliminary treatment units?

A very important aspect is the removal of sanitary litter e.g. sanitary towel strips, condoms, cotton and sticks. Very unsightly, block’s equipment, major PR problem in sludge use.

Paper, rags, pieces of timber, etc. arriving through the sewers at the wastewater treatment works can block pipes and damage machinery and processes. This material is removed by screens at the inlet to the works. Frequently contaminated with sewage solids, screenings should be separately stored, preferably after a washing process and strained of excess water. They should either be burnt on site in an appropriately designed incinerator, or carted into the sanitary landfill used for the disposal of solid refuse.

Sand and grit enter sewers principally from roads and can damage machinery and accumulate in process pipework. In order to avoid this, and to protect the works, sand and grit, are removed in one of the first treatment units. Sand and grit should also be disposed of to a sanitary landfill. If units removing the sand and grit are operating correctly, the material should be inoffensive to handle but, if not, washing prior to disposal may be necessary.


Fig. 2.7 Grit and sand is disposed of to sanitary landfill