B. Removal and treatment of wastewater

Introduction

"Nothing is lost, nothing is created, everything is transformed.." This principal applies equally to water: it is the source of life, much effort is spent to get it; but it also is a source of death, and it is essential for health that the same effort is made to remove it after use.

Health risks and nuisance

These risks are due to organic and biological pollution carried by wastewater as well as the presence of stagnant water:

- breeding of insect vectors (anopheles, culex);
- spread and multiplication of pathogenic agents such as cholera vibrios and schistosomonas, etc.;
- chemical contamination of water (nitrates, detergents) and ecological disturbance of aquatic environments;
- production of noxious and corrosive gases.

Definition of wastewater

The risks associated with wastewater depends on its origin, and it is useful to classify the important sources:

- Domestic wastewater

· Sewage: water carrying excrete in suspension, thus containing bacteria, viruses and fl parasites and also nitrogen.
· Sullage: water from the bathroom, the kitchen, laundry, etc., containing detergents and fats as well as micro-organisms of fl origin.

- Agricultural wastewater

· Stockraising effluent: slurry and manure.
· Crop growing activities: fertilizers and pesticides.

It is usual to measure the degree of pollution by the following parameters:

- Daily volume of effluent.
- Chemical Oxygen Demand (COD): a measure of the total organic content.
- Five day Biological Oxygen Demand (BOD5): the organic content biodegradable within 5 days.
- Total Suspended Solids (TSS).
- Nitrogen content (ammonia and organic nitrogen).
- Phosphorous content

General principles

For every place supplied with water there should be a removal system which prevents stagnant water and local pollution.

Treatment methods aim to fix the chemical and biological pollution (by sedimentation, filtration, etc.), and/or to destroy it by biological, chemical or physical processes and then to dispose of the treated water by infiltration into the ground, or into surface water (river, lake sea, etc.).

This field may become very technical and involves special expertise which is beyond our capabilities.

In practice, the problems faced in the field are few and generally simple to solve; for instance:

- stagnant water around a water point: well, tap, etc.,
- washing areas: bathing, cooking, laundry, etc.,
- laboratory and health centre wastes, etc.

More rarely:
- flushing toilets,
- house or hospital sewers.

Removal

A removal system should be able to remove wastewater, so as to avoid stagnant water, and to channel it to the disposal or treatment site without contaminating the local environment.

The collection surface should be gently sloped (1%) and cemented. Before removal, it may be necessary to pretreat the water to remove solid or dissolved matter which could hamper the removal and final treatment (see technical briefs):

- Grease-trap to eliminate fatty material which might block channels.
- Screen to remove floating objects.
- Sedimenter or sand trap to separate sand, soil, etc.

These structures become ideal vector breeding sites if they are not well maintained.

The removal system design may be based on different techniques:

- Open channel

This is the most simple and least costly technique but it entails maintenance problems: blockages, stagnant water, damage to the sides, etc.

This technique should be used only for drainage of rainwater or of wastewater over short distances; the channel should be cement-lined if possible and with enough slope to be self-cleansing

- Gravel drain

The open channel may be improved by lining it with plastic sheeting, filling it with coarse gravel, covering it with more plastic sheeting and then with earth.

The wastewater should never contain suspended material capable of blocking this type of drain which is impossible to unblock. This technique may be used in an emergency, for example at a dispensary or a laboratory.

- Pipe drain

This is the most effective way of removing wastewater but also the most costly.

Various types of pipe may be used (PVC, polythene, cement, fibrocement, etc.), with a minimum diameter of 100 mm.

The slope and the pipe diameter should be adequate for the flow, and the pipes should be buried correctly so as not to be destroyed by the passage of heavy vehicles (20cm of compacted earth minimum).

Check regularly in order to spot and deal with blockages.

Treatment

Wastewater treatment techniques mostly need specialised skills and technologies.

For this reason, these sophisticated techniques will not be studied in this guide. Information will be limited to infiltration systems and the basic principles of waste stabilization ponds.

INFILTRATION

Infiltration uses the natural capacity of the soil to fix particles present in water by filtration, and to purify the water by a process of biological decomposition capable of destroying micro-organisms and chemical pollution.

This natural capacity is always extremely variable, depending on the soil type:

- A mature organic-rich soil is host to intense biological activity favouring purification, but it blocks rapidly and so has a reduced infiltration potential;
- Conversely, a sandy soil may have an infiltration rate which is too rapid and which does not allow sufficient time for purification if the water table is too close to the surface;
- For the same reasons, a fissured rock would have only a small purifying capacity.

In practice, the two following parameters should be studied:

- The slope of the ground: a slope too steep may encourage water to reappear and so contaminate the ground.
- The infiltration rate: determined by percolation tests with clean water (see technical brief Soil permeability).

The principle of infiltration is used in the following techniques:

· soakaway pit,
· infiltration trench,
· evapotranspiration area,
· irrigated garden.

WASTE STABILIZATION PONDS

Waste stabilization is a biological process which takes place in ponds arranged in series.

It is an effective technique for the elimination of pathogens and is relatively easy to maintain, but the design and implementation should be left to specialists, or the result may be an almost insoluble problem.

It may be assumed that with a series of three ponds and a retention time in the ponds of 11 days, a reduction of 99.9% in the number of fl germs may be achieved.

The reuse of wastewater for irrigation after treatment in ponds may be useful, provided that the following rules are followed:

- ensure that irrigation is not likely to create areas of stagnant water;
- irrigate crops which are not in contact with the soil (e.g. fruit tree), or which are cooked before eating.