Biological wastewater treatment processes and the use of microbes
In biological processes, various types of microbes and bacteria are used that digest the waste materials while injecting oxygen into the water with the use of aerators. In order to separate the contaminants from the water, growth of the microbe population is brought to a point that will be sufficient to digest the pollutants in a given time span. The basic conditions for the vitality of these aerobic microbes are the supply of organic nutrients found in the wastewater and oxygen. Following the conclusion of the digestion stage, the microbes develop into larger blocks that can be separated from the liquids by settlement, after which high-quality effluents are obtained. As noted, the entire biological treatment process is based on an activated sludge process – this is a process whereby a dense population of these microorganisms is held in a high and continuous state of activity in concrete ponds that are planned for this purpose (biological reactors) into which filtered raw wastewater is fed. The microorganisms decompose carbon and nitrogen compounds that are the principal components of municipal and industrial sewage. In order to maintain the continuous activity of the microbiological population (the biomass) and achieve proper treatment, the microorganisms in the wastewater stay in the biological reactor for an average period of 12 to 14 hours.
During that time, the biomass is enriched with oxygen that is recycled into the biological reactor by mechanical means (horizontal aeration units) and continues to be mixed by circulation within the reactor. Before reaching the biological reactor, the raw sewage undergoes pretreatment, which includes filtration through a two-stage system of bar screens in order to remove non-decomposable suspended solids and grit chambers in order to remove sand and floating fats. All of the materials that have been separated by filtration, sand settlement, fat flotation and [screening of] other inorganic materials are removed in closed metal tanks [and brought to] to disposal sites away from the treatment plant. After leaving the biological reactors, the mixed liquid, which is comprised of treated effluents and floating biomass passes into sedimentation basins, the purpose of which is to separate the mixed liquid components into effluents with minimal turbidity and thick activated sludge (biomass), which is mostly recycled into the process and partially removed as excess biomass for disposal at sea. The wastewater is treated at Shafdan by accelerating natural biological processes that bring about the removal of organic materials in the wastewater and their decomposition. In order to do this, “waste-eating” microbes are propagated in the water that digest the waste materials in the wastewater and decompose them. In order to propagate the microbes, the wastewater is enriched with oxygen, which is forced into it through a waterworks.
At the first stage, hard wastes such as boards, plastic bags and rags are removed from the wastewater. This is done with the use of bar screens (filters). From there, the wastewater passes into a grit sedimentation installation from which other fats, stones and sand are removed that settle to the bottom of the installation.
Biological aeration tanks (reactors)
Following the pretreatment process, the wastewater passes into aeration tanks (ventilated reactors). At the conclusion of the waste digestion process, the microbes are separated from the water in separation tanks and the treated water is pumped from the upper part of the tanks. Shafdan has 4 aeration tanks, each with a capacity of 55,000 m³. Each aeration tank is equipped with 36 double aerators, the function of which is to force air (oxygen) through the wastewater and to push the wastewater within the tank in order to obtain better flow of air within it. The regular flow of oxygen accelerates the development of the waste-eating microbes. The raw wastewater, with the addition of the mass of microbes (sludge), stays in the aeration tanks for about 12 hours. At the conclusion of this process, the liquid enriched with the “satisfied” microbes are sent to settlement basins (clarifiers).
Settlement basins (clarifiers)
Shafdan has 12 settlement basins, each with a capacity of 7,500 m³. The microbe mass (the sludge) is separated in these tanks from the water by a physical sedimentation process. The clarifiers are equipped with rotating bridges to which rakes are attached, which move on the bottom of the tanks and pick up the sludge that has settled at the center of the tank. This process continues for about five hours, and when completed, about 40% of the sludge is recycled into the biological aeration tanks (reactors) for further wastewater treatment, while the rest of the sludge is discharged into the sea. Discharge into the sea is performed through pipes, the outlets of which are located about 5 km from the shoreline and at a depth of about 40 m. It is important to note that this discharge is monitored frequently by the Israel Oceanographic and Limnological Research Institute and that, according to the results of these tests, no serious or irreversible damage is caused to the marine environment. Following treatment in the settlement basins, the wastewater becomes effluent. The effluent is transferred to a reclamation plant where a final treatment process is performed in which the effluent is discharged into fields of sand and the water is stored in the aquifer.
The Mekorot Water Company, to which the effluents are transferred, injects them into sandy soil. Sand is considered to be the best kind of natural filter, and is used as a final filter for the water. The water percolates into the ground over a period of about 400 days through the fine filtration provided by the dense grains of sand and settles down to a solid substrate. At the conclusion of this period, the water can be used as top-quality irrigation water that is very close in quality to drinking water, and is pumped and fed for farm irrigation in the Negev.