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close this bookGuide to the Development of On-Site Sanitation (WHO, 1992)
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View the documentPreface
Open this folder and view contentsPart I - Foundations of sanitary practice
Open this folder and view contentsPart II - Detailed design, construction, operation and maintenance
Open this folder and view contentsPart III - Planning and development of on-site sanitation projects
View the documentReferences
View the documentSelected further reading
View the documentGlossary of terms used in this book
View the documentANNEX 1 - Reuse of excrete
View the documentANNEX 2 - Sullage
View the documentANNEX 3 - Reviewers

ANNEX 2 - Sullage

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 excrete).

There are few published studies of the characteristics of sullage in developing countries. Research in the United States of America has shown that sullage has a lower nitrate content than toilet wastes, and a more soluble and more biodegradable organic content (Leak, 1974). The suspended solids load in sullage is lower than in wastes from toilets, but it contains more grease and is generally at a higher temperature. Kitchen wastes have a higher suspended solids content, a higher biochemical oxygen demand, and a higher nitrate concentration than other sullage.

The volume and characteristics of the sullage produced by one community may be very different from those of another. A family served only by a remote standpipe or handpump may discard less than 10 litres of sullage per person each day, whereas members of a household with numerous plumbing fixtures may discard 200 litres each or more per day. In some countries, rivers or lakes are used for personal hygiene and for washing clothes and utensils, so that the volumes of wastewater leaving the house are low. Table A2.1 shows the water consumption measured in rural households, demonstrating a wide range of consumption rates.

The nature of the sullage is markedly influenced by factors such as diet, methods of washing clothes and utensils, habits of personal hygiene, and the existence of bathrooms and other facilities.

There are several reasons for keeping sullage separate from excrete. First, there may be a system for on-site disposal of excrete that cannot accept large volumes of water. Alternatively, the sullage may be transported away from the site by a small-diameter pipe that could not handle faeces. A third reason might be to reduce the hydraulic loading on a septic tank by diverting the sullage away from it (Bradley, 1983).

Sullage is discharged or disposed of in a number of ways. Often it is simply tipped on to the ground in the yard or outside the property where it evaporates or percolates into the soil. It may be used to irrigate a vegetable or flower garden. It may find its way by natural or designed routes into open or subsurface storm drains. Soakpits or drainage fields may be built to disperse the sullage. In some cases the greywater from a number of properties is collected, screened and treated in ponds before it is discharged or reused.

Table A2.1. Water consumption (litres per person per day) in some rural areas in four developing countries

Water use

Lesotho a

Uganda b

Pakistan Punjab c

Mozambique d



Lango

Kigezi



Drinking and cooking

80

58

64

57

23

Other domestic use

100

119

16

24.0

10.0

Total

18

18

8

30

12

a Feachem et al. 1978
b White et al.. 1972
c Ahmed et al, 1975
d Cairocross S, personal communication.

Health implications of sullage management

In general, the health hazards posed by sullage are not as serious as those associated with either 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 (Bradley, 1983), 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 (Hypes, 1974).

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.

It is often thought that the provision of a more abundant supply of water to a community will necessarily bring about an improvement in health. However, if the greater availability of water causes the creation of pools of stagnant sullage (because sullage disposal has not been carefully considered), then the improved water supply could have a negative effect on the health of the community, largely as a result of the increase in the mosquito population. The disposal of sullage is a particular problem at communal water points. Often, large volumes of wastewater are generated and, if provision is not made for its proper disposal, a significant health hazard may develop.

Disposal of sullage

Pouring sullage on to the ground can be an acceptable means of disposal provided that the soil is not continually moist. This means that the soil must have sufficient permeability and be of adequate area to allow the sullage to percolate away. This method of disposal can be put to good effect by using the sullage to irrigate vegetable gardens, but vegetables that will be eaten raw should not be watered with sullage because of the danger of disease transmission.

Infiltration through field drains or soakpits, as used for the disposal of septic tank effluent, is suitable for sullage. The size of soakpits and trenches may be designed using the long-term infiltration rates shown in Table 5.4. Examples 8.6 and 8.7 in Chapter 8 explain the design of infiltration systems.

Sullage often finds its way, by accident or design, into open drains. Such drains can be a satisfactory method of conducting the sullage to a body of receiving water provided that there is no ponding of sullage in the drains. Pools encourage mosquito breeding, and children often play in them. Ponding is likely to occur where the terrain is flat and the drain slope small, where the drains are rough and unlined so that water collects in depressions, where refuse is deposited in the drains, and where drains are filled in to allow vehicles or pedestrians to cross.

Storm drains that are also used for transporting sullage should have a compound cross-section as shown in Fig. A2.1. This is because the flow of stormwater in the rainy season can be hundreds of times larger than the flow of sullage alone. A simple cross-section designed only for stormwater would conduct the sullage away at a very low velocity, leaving the solids suspended in the sullage in the bottom of the drain. The circular channel in the invert of a compound cross-section allows small flows to move at a higher velocity. Drain-cleaning tools should be adapted to fit the small central channel.


Fig. A2.1. Cross-section of a compound storm drain

Keeping drains free of refuse is not easy. Unfortunately it is commonly believed that a drain is an appropriate place for depositing solid waste, especially where there is no adequate refuse collection service. Refuse in drains quickly becomes malodorous as it decomposes and is attractive to flies as a site for egg-laying. Removal of such material from the drains is not a popular task. The problem of solid waste in open drains calls for a three-pronged attack:

- the provision of a satisfactory refuse collection service to provide an alternative outlet;
- public education, especially on the need to keep the drains clear; and
- vigilance on the part of municipal labourers to remove blockages wherever they occur.

In one city in Brazil, each householder was made responsible for keeping the length of drain outside his or her property clean; grilles were fitted at points in the drains in line with the dividing walls between the properties so that no refuse could be carried on to a downstream neighbour's section, and any flooding occurred in the area where the refuse was deposited (Cairncross, personal communication). Grilles of this type have also been used in other countries, installed by the municipal authorities or by residents to prevent refuse entering the sections of drain for which they are responsible.

Covering the drains may appear to be a solution, but if refuse is deposited in the drains through gaps or by lifting cover slabs, the resulting blockages and ponding are much harder to detect and clear.

Small-bore sewers can be used to convey sullage. Diameters and slopes can be less than those recommended for sewage containing excrete because the solids load is less. Where sand is used for scouring cooking pots it may be necessary to install traps to collect this grit before it goes into the sewer. However, attention must be given to ensuring that these traps are emptied periodically - the mere provision of them without adequate maintenance is not enough. There may also be a risk of grease deposits building up in the pipes. Grease traps can be used to separate grease from the rest of the wastewater, but they are only effective if the accumulated grease is removed at regular intervals. Generally, grease traps are fitted at garages, restaurants and other commercial premises where large quantities of oil or grease are discharged in the wastewater (Fig. A2.2).

Sullage can be treated on site to make it more acceptable for final disposal or reuse. Septic tanks can be used; they are effective in removing grease and solids, and do not require frequent desludging (Brandes, 1978). Intermittent sand filters are effective in reducing biochemical oxygen demand and nitrate levels, but Boyle et al. (1982) found they had little effect on the numbers of faecal indicator bacteria.

The selection of the most appropriate sullage disposal system depends on many factors, such as rainfall, soil structure, topography, housing density, water consumption, latrine type, and a variety of social and economic factors. For example, where there is sufficient yard area, the soil is permeable, the rainfall is such that ponding never occurs, and sullage is produced in small quantities, it may be quite satisfactory to pour the sullage directly on to the soil. Where the subsoil permeability, housing density and income permit, a soakpit is recommended. Alternatively the sullage could be disposed of in a pit latrine, where one exists. Where there is sufficient slope for surface drains, and the ability to keep them free of debris has been demonstrated, disposal to these drains might be acceptable as an interim measure, provided the drainage system has an appropriate discharge point. Small-scale pilot studies are often valuable for assessing the suitability of the various alternatives before implementation on a large scale.


Fig. A2.2. Cross-section of a grease trap

The main problems posed by sullage are socioeconomic rather than technical in origin. Most disposal systems only function correctly if operated and maintained in a proper fashion. This is particularly evident in surface drainage systems where the agencies responsible for maintenance are often under-funded and thus unable to carry out their duties adequately. In such circumstances, the maintenance must be taken over by the community - but the community must first be convinced that clean drains are necessary for good health.

References

AHMED, K. ET AL. (1975) Rural water consumption survey. Lahore, Institute of Public Health Engineering and Research (Report No. 026-12-74).

BOYLE, W. C. ET AL. (1982) Treatment of residential greywater with intermittent sand filtration. In: Eikum, A.S. & Seabloom, R.W., ea., Alternative wastewater treatment. Dordrecht, Reidel, pp. 277-300.

BRADLEY, R. M. (1983) The choice between septic tanks and sewers in tropical developing countries. The public health engineer, 11 (1):20 28

BRANDES, M. (1978) Characteristics of effluents from grey and black water septic tanks. Journal of the Water Pollution Control Federation, 50 ( I 1): 2547 2559.

HYPES, W. D. (1974) Characteristics of typical household greywater. In: Winneberger, J. H. T., ea., Manual of greywater treatment practices, Michigan, Ann Arbor Science, pp. 79 88.

LAAK, R. (1974) Relative pollution strength of undiluted waste materials discharged in households and the dilution waters used for each. In: Winneberger, J. H. T., ea., Manual of greywater treatment practices, Michigan, Ann Arbor Science, pp. 68 78.

WHITE, G. F. ET Al.. (1972) Drawers of water. Chicago, Chicago University Press.