9.   WASTE WATER TREATMENT

Dwelling houses and farms which have their own water supply and sewage system produce on average 30 - 200 liters of wastewater per person a day, depending on the living standards of the family and the level of development of the territory. In case of industrial production, industrial wastewater is added.

Permit of special use of water is needed in the following cases: if ground water is extracted from very deep layers, or if ground water is extracted from other layers and the amount of extracted water exceeds 5 m³ per day, or if the amount of water extracted exceeds 30 m³ per day, or if wastewater is discharged out of the boundaries of the property.

Problems of treatment of wastewater originating from agricultural point pollution sources and rainwater are described below:

In Estonian conditions wastewater originating from farms and agricultural production enterprises can be divided into the following groups:

Leakage water which forces its way into sewage pipes, sewage wells and sewage water pump stations which are not watertight adds to the total amount of wastewater.

The surroundings of the residential and production buildings of a farm should be planned so that clean rainwater running off the roofs and compacted or covered areas would leach into the surrounding green areas and soak further into the soil.

Connecting single dwelling houses situated far from settlements to the public sewerage system could be too expensive. In this case wastewater has to be collected into a cesspool and transported from there to the wastewater treatment facility of this settlement.

Another possibility is to purify wastewater within the boundaries of property and discharge it then into a recipient or let it soak into the soil. In the latter case the valid legal acts must be observed in accordance with the conditions set in the water permit. It is obligatory to consult hydrogeologists and specialists in the relevant field.

Permit for special use of water is not necessary for discharging the domestic wastewater from personal household into the soil but this activity must be carried out in accordance with certain requirements.

It is prohibited to discharge sewage water into the groundwater and wastewater onto the frozen soil. (Water Act)

The pattern of purification of sewage water in the soil of on a personal plot depends on the water permeability of the ground as well as on the level of groundwater and the rate of protection. In each single case the solution acceptable from the point of view of water protection must be found, spoiling of either surface or groundwater is prohibited. Before the wastewater will be soaked into the soil, it is advisable to consult the county environmental authorities. They will, on the basis of the amount of wastewater and the degree of its pollution estimate and decide, whether the permit for special use of water is necessary. They also recommend how to manage wastewater treatment in an environmentally friendly manner.

On the treatment of sewage water from small sites the following solutions are used:

The location of soil filtration site in relation to a drinking water well must be chosen very carefully. It takes 2 - 3 months for most of pathogenic bacteria to be destroyed in the soil. This means that wastewater soaked into the soil should reach the seeping well during that time. The seeping system must be situated downhill in relation to the well and downstream in relation to the direction of groundwater flow. If there are no exact data the distance between the seeping system and the well can be determined by the soil type and the surface incline:

* d₁₀ - diameter of grain compared with which only 10% of particles in the ground are smaller.

Figure 7. The sewage water mechanically pre- treated in the septic tank will either be soaked into the soil or purified in the seeping system.

The minimal distance of the seeping system from roads, footpaths or the boundary of the plot is 5 m and from water bodies or ditches 10 - 30 m (the bigger the surface incline the longer should be the distance). The distance of the septic tank or the biological treatment facility from the house or plot boundary should be at least 10 m.

9.1. Individual sewage treatment facility  

A cesspool toilet is the simplest form of individual sewage treatment facility. If every time after the cesspool toilet is used, a scoopful of peat or sawdust is strewn over the excreta, the anaerobic fermenting will be replaced by odourless composting. It is also possible to build or purchase a special compost-based cesspool toilet.

Irrespective of the type of individual sewage treatment facility the first section of it must always be a septic tank. The capacity of the septic tank depends on the number of users and the type of the toilet (WC or cesspool toilet). The capacity of the septic tank for a normal size family (up to 5 persons) should be 4 M³ for a dwelling house with WC, and 2 m³ for a house with a cesspool toilet. As usually 20 - 30% of all wastewater and a great part of household pollutants (20 - 60% of organic substance, 20 - 40% of phosphorus, 70 - 90% of nitrogen, about 60% of suspended solids and most of the pathogenic microbes) come from water toilet, it would, from the point of view of protection, be rational to give up WC and replace it with the cesspool toilet. When using the cesspool toilet, expenditures for building sewage treatment facilities are considerably smaller as it is much easier and cheaper to purify the so-called grey water.

The technical means for seeping domestic wastewater into the soil are the seepage trench (system of seepage trenches), seepage bed and leaching cesspool. The leaching cesspool can be used only for purifying gray water of one household. The length of seepage trench (system of seepage trenches) for a household of 5 persons (width of the bottom of 1 m) depends on the speed of seepage:

A treatment system, which is based on soil filtration (filter trench/system of trenches, sand filter bed or filter well) is built if the seepage system cannot be used (water absorbing capacity of the soil is low: speed of seepage < 2 m/day) or must not be used. The water, which has permeated the filter, will be discharged into the trench, drains or natural water body. The only difference between the sand filter bed and the system of filter trenches is that parallel distribution pipes (the distance between them - 1.2 - 1.5 m) and the drains are situated in a common bed similarly to seepage bed. A filter well can be used only periodically and for the purification of grey water only, if the area is too small for filter trenches to be built. The calculated load of a filter well is 100 - 125 l/(m² x day).

The wells and pipelines of sewage treatment systems, which are based on seepage water, may be built of local materials, but the best results can be achieved by using plastic elements.

9.2. Treatment of waste-water which comes from producing and processing of agricultural products

Wastewater discharged into the sewage system from production enterprises may be of very specific content and uneven flow rate depending on the efficiency of production and the technology used. Thus, individual approach is needed at every singular site. The schemes for purification of industrial wastewater must be drawn up simultaneously with planning technological solutions for primary production. When treating concentrated industrial wastewater it is sometimes practical to dilute the water discharged into the wastewater purification facility with purified water or domestic wastewater.

Wastewater originating from processing plant products, contains large quantities of carbohydrates (sugar, starch), water originating from animal husbandry products contains also fats and proteins and often also large quantities of animal fodder. As wastewater originating from agricultural production contains organic substances, it is possible and widespread to use biological treatment.

In farms industrial wastewater is produced on washing the equipment and the rooms and domestic wastewater comes from sanitary facilities. The amount of domestic wastewater is usually 20 - 25 I/per person a day and the pollution load 0.5 human equivalents (he). Sometimes emergency slaughtering has to be carried out and sometimes it happens that milk (also colostrum) is poured into the sewage system. If blood is discharged into the sewage system, the amount of sewage water calculated per one bovine animal is 0.6 m³ and the pollution load 200 he. In the case of pigs the respective figures per one pig are 0.4 M³ and pollution load 100 he. Usually the amount of colostrum poured into the sewage system all at once is 20 - 40 l and the pollution concentration in this case is more than 1,000 times higher than in the case of domestic wastewater.

Such a sudden increase of pollution load may cause breaking down of the purification facility. Collecting blood, colostrum and other substances and preventing the discharge of them into the sewage system can avoid sudden pollution. For farms more extensive installations for wastewater should be preferred: e.g. stabilization ponds with an aerated first stage or soil filtration facilities. If hydro geological conditions do not allow this, biofilters can be used. Biofilters can withstand big fluctuations in hydraulic and pollution loads and there is no danger that activated mud can be washed out during peak hours. It is recommended to consult experts in the field and specialists of county environmental authorities before establishing a wastewater treatment facility.

In dairy farms industrial wastewater comes from milking rooms. When a farm has a milking parlour, wastewater from the milking parlour containing manure, disinfectants and combined fodder is added to the rest of water. The amount and level of concentration of this water depends on the size of the farm, the equipment of the milking parlour, the size of milking room and the waiting area, the methods of udder washing and the general working habits of the farm workers. Wastewater discharged from a dairy farm should be at first purified in local wastewater treatment facilities (mud and manure trap connected with the flow unifier) and then, if possible, discharge it to the public sewerage system of the settlement. In case the dairy farm is situated far from the settlement, the wastewater should be mixed with domestic wastewater in a flow unifier and discharged into the biological purification facility.

Wastewater discharged from a milking room contains only a small amount of nutrients, but too many chemicals. In this case formation of active mud in the purification facility may not take place (disinfectants and detergents inhibit the active mud processes) or be very thin. Also depositing processes of active mud are blocked and it may be carried out of the facility. The situation can be solved only by very efficient use of chemicals and environmentally friendly choice of treatment facilities.

In pig breeding farms industrial wastewater comes from the fodder-kitchen, where potatoes and feeding equipment are washed. If pigs are kept on dried fodder, there will be no wastewater. The wastewater from pig breeding farms has to be purified on the spot in the mud-and-manure trap and then discharged into the flow unifier. Thereafter follows biological purification and only then the wastewater may be discharged into the main wastewater treatment facility.

In poultry houses the amount and composition of wastewater depends on the species of poultry and the methods of keeping these birds. When hens are kept on the floors, wastewater comes only from cleaning, washing and disinfecting poultry houses between the cycles. Usually this water contains big amounts of fodder. If there is a slaughterhouse section in this poultry yard, wastewater must be cleaned from feathers, using racks, sieves and sometimes also grease traps before discharging into the sewerage. Wastewater from poultry houses can be purified only in stabilization ponds or soil-based filtration facilities because no other type of treatment facility can withstand such big fluctuations in flow rate. Before discharging the wastewater into stabilization ponds, it should be treated mechanically in a septic tank or a compound settling basin (Emscher tank, a two-storey settling basin). For hen houses it is advisable to have both local purification as well as biological treatment facilities planned simultaneously.

In slaughterhouses and meat-processing enterprises wastewater comes from washing raw materials, sausages, equipment, packaging and floors. The water contains grease, little pieces of meat, blood, proteins, salt, nitrites and saltpeter. The concentration of pollutants is high, the amount and the pollution load of the wastewater depend on the amount of water used, the level of waste products caught and utilized, and on the general working habits of the workers.

Mud and grease traps are obligatory in local treatment facilities of slaughterhouses as well as in other enterprises where meat is processed. After passing through the local treatment, wastewater may be purified in all types of biological treatment facilities. As the concentration of pollutants in that water is high, it would be better to purify it after it has been mixed with domestic wastewater. Otherwise the treatment procedure must consist of two phases.

In enterprises where poultry are processed, wastewater comes from slaughtering and plucking the birds, cleaning the carcasses, washing the equipment and the rooms. This water contains feathers, blood, fodder, sand, manure, and small quantities of grease. The pollution load of the enterprise depends on whether blood is collected or discharged into the sewerage system. Wastewater, which has been cleaned from feathers, fat, and slaughter residues on the spot, may be discharged into public sewage system or treated in the purification facility of this enterprise. The biggest problem with poultry-processing enterprises is the feathers, which can completely block the wastewater treatment facility.