Wastewater Treatment Plants

Small treatment plants apply the same engineered processes and technologies as medium or large plants, but in most cases of small treatment units the different processes are combined in a compact tank with different compartments. The main goal of the treatment is removal of suspended solids, organic materials and in some cases – nutrients. With well-designed and maintained units, the effluent could be suitable for discharge into natural water bodies or dry ditches, and also, for different types of beneficial use. The most important part of the treatment process is the biological removal of organic material, which depends on the development and maintenance of a population of different bacteria and micro-organisms, known as biomass. These living micro-organisms require specific environmental conditions, such as temperature, specified values of pH and other water quality parameters.

Aerobic solutions

In the biological treatment processes presented below, the bacteria are aerobic, meaning that they require oxygen for their survival (they need to breathe). The biomass uses the organic materials in the wastewater as food for their growth and reproduction. What can kill the biomass and jeopardize the treatment process?

· Lack of food – no influent wastewater for a long period

· Lack of oxygen – the supply mechanism is not working

· Toxic elements in high concentrations – bleach and other cleaning agents; acids; basic compounds; salts; toxic products as insecticides and pesticides, etc.

· Low temperatures close to freezing for prolonged period of time

In summary, the best way to maintain the plant in good condition is to provide regular flow of wastewater and to avoid using the toilet and the sinks for disposal of toxic and damaging fluids, as well as, plastic, paper, glass, metals and other similar materials.

In order to separate the biomass from the treated wastewater and to clarify it, there is provision of a separate chamber (sedimentation tank), where the sludge (biomass) is separated from the water and maintained in the system or removed from it at certain intervals of time. The quantity of excess sludge produced, and correspondingly the time intervals for its removal, depends on the type of process applied and should be mentioned in the O&M manual of the plant. In the latest technological developments, instead of sedimentation chamber, a membrane separation unit can be applied.

There are a large number of different types and modifications of small treatment plants but they can be grouped in two basic categories: attached growth systems and suspended growth systems.

It is important to note that a proper functioning of such a plant requires regular load and adequate air supply. Unfortunately, decentralized and more specifically on-site applications, are producing an extremely varying load due to the low number of population served. In many cases, it is possible that the plant might work very well from mechanical point of view. But the water would only pass trough with very little treatment effect, due to disturbance of the biological process. This is very likely in cases of seasonal occupancy or large periods without occupants living in the residence. For these reasons, the maintenance of such treatment facilities by professional O&M companies is highly recommendable.

Anaerobic solutions

Wastewater treatment technologies applying anaerobic bacteria for the biological treatment process are available as well. With this treatment technology there is no need to supply oxygen for the performance of the biomass and therefore their operation is easier. However, the level of degradation of the organic matter is lower - 50-60%, and for this reason the effluent can not be disposed in surface water without additional biological treatment. In most cases, the anaerobic technologies are in fact an improvement or enhancement of the septic tank. These technologies might be suitable in warm climates, because the biomass requires higher temperature in the reactor to perform adequately. The limitations described in the section "aerobic solutions", regarding the food supply and toxic elements are valid here as well. In general, this type of technology performs well with higher temperatures and more concentrated wastewater, and also, it has the advantage to produce biogas, which could be collected and used for beneficial purposes.

Examples of anaerobic treatment technologies are the UASB (Up-flow Anaerobic Sludge Blanket) reactor and the anaerobic baffled reactor.

Stabilization ponds

This treatment technology is applied often, especially in warm climates with mild winter. It resembles closely the natural process of biological degradation in lakes. The stabilization pond system consists mainly of three types of ponds. First is the anaerobic pond, which collects the main part of the suspended solids. The accumulated sludge is decomposed by means of anaerobic digestion. In some cases of medium to low concentrated wastewater the anaerobic ponds might be omitted. The second type is the facultative pond, where the major biological degradation takes place by both aerobic and anaerobic means. The third type are the maturation ponds, which have a polishing function and provide a very good level of disinfection. The air necessary for the aerobic biological process is supplied by the water surface, and for this reason, the depth of the ponds is low, but large area is necessary for the treatment plant. Stabilization ponds require low O&M efforts and costs.