1.3 Methods of processing sewage
Sewage can be processed with three principal methods: mechanical, chemical and
biological. The sewage treatment is usually a combination of the three principal methods, such as
mechanical-chemical, mechanical-biological and chemical-biological. The treatment of
sewage includes the following stages:
1. Waste water accumulation and management
In this stage of processing waste water, black water, grey water and galley water have been collected in holding tanks before the processing plant.
2. Waste water pre-treatment
Wastewater pre-treatment protects the other phases of the purification process. Sewage
contains a lot of solid waste and grease that may cause problems in the later stages of the
process. The pre-treatment process reduces the amount of solids in the waste water. Effective
waste-water pre-treatment also reduces the need for oxidation. The pre-treatment is
mechanical and consists of sieving and sedimentation units. The large particles pass through a
shredding pump before sieving.
3. Waste water oxidation
The mechanical filtering results in a maximum of 50% reduction in organic load. The remaining organic compounds have to be oxidized, either chemically or biologically. Certain chemicals, e.g. ozone, chlorine, hydrogen peroxide, are added to the sewage in the chemical oxidation. The chemicals oxidize the organic impurities in the sewage water. When compared to the ozone and hydrogen peroxide, chlorine is not a very environmental friendly oxidant because of the carcinogenic compounds that develop as a by-product of the reaction. The added chemicals have an impact on the organic matter that has dissolved slightly and the BOD reduction remains small. The estimated treatment results for reduction in BOD and phosphorus are good. “Over-chlorination” results in high levels of residual chlorine in the discharge, which is lethal to marine organisms. In the biological treatment the micro-organisms use the impurities in the sewage as their nourishment. There are several types of bioprocesses and the most common biological process is the active sludge treatment plant, where the sewage is mixed in a continuous-action aeration tank with active sludge. Biological filters and biorotors are also used as biological treatment plants. In these devices the bacteria that destroy the impurities attach to the filtering material. The biological treatment system is the most efficient way of reducing the BOD load. The estimated reduction in BOD is 80–95% and the reduction in phosphorus is 20–40%. The effectiveness of the bioprocess depends on the amount of active biomass and the bacteria living conditions. The disadvantages of biological treatment are the long starting period and its sensitivity to external disturbances. The reasons for malfunction of the biological system are the following: Strong chemicals that have got into the plant are destroying the bacteria, Bacteria die due to the lack of oxygen when the ventilation does not work, The return of active sludge does not work.
4. Waste water clarification and filtration
After oxidation, the sludge is separated in a sedimentation tank and returned to the aeration tank. Separating the active biomass, sediment particles and bacteria from the water is a critical phase in the wastewater purification process. The clarification and filtration processes used in the ships are membrane filtration, dissolved air flotation (DAF) and settling. The DAF system relies on the injection of microscopic air bubbles into the feed water stream, causing the particles to float on the surface of a basin with inclined settling plates, from which they are continuously skimmed off and removed with a wastewater stream. It is useful when treating waters that are high in Total Suspended Solids (TSS) or have highly variable suspended solids content.
5. Waste water disinfection
The last phase in the wastewater purification process is disinfection. Depending on the previous treatment method, the disinfection enhances the quality of the wastewater or is an essential part of the purification process. When the membrane clarification and filtration is used, the disinfection is performed with UV-light. If the water is very turbid, the UV-light is not suitable for disinfection. The other potential disinfectants are, for example, chlorine, radicals and ozone.
The sludge production depends on the treatment process. Effective pre-treatment before
the bioreactor reduces the sludge production and enhances the sludge drying. The sludge that
comes straight from the process is centrifuged. The centrifugal treatment raises the drysubstance concentration to 17–27%, compared to the before treatment dry substance concentration of 2–3%. After the decanter centrifuge, the possible sludge handling techniques are
holding, incinerator, steam dryer, filter press or an alternative sludge conditioning process so
that combustion is possible.