The effects of pressure on aerobic biological wastewater treatment using rotating biological contractors

The aim of this research was to investigate the application and effects of pressure on an aerobic biological wastewater treatment process. For the purpose of the investigation two specially designed, identical, laboratory-scale rotating biological contactor (RBC) units were constructed. One of these was held in a steel pressure vessel while the reference unit was operated open to the atmosphere. The treatment capabilities of the pressurized unit, as compared to those of the reference unit, were determined for a variety of organic loadings at increasing pressures up to a limit of 6 bar. During the investigation, the substrate employed was a synthetic wastewater made up frequently in the laboratory. Most of the analytical work was carried out on composite druly samples of the feed wastewater and of the two effluents produced, both filtered and nonfiltered. In addition, sludge samples from both units were regularly tested for a variety of sludge parameters. The water quality parameters investigated were the 5-day Biochemical Oxygen Demand (BOD5), the Chemical Oxygen Demand (COD), Suspended Solids, pH, Temperature, Ammoniacal Nitrogen and Total Oxidized Nitrogen (TON). The yield of sludge produced and certain characteristics of sludge i.e. Specific Stirred Volume Index (SSVl). Specific Resistance to Filtration, were also obtained. The quantity of air required for the pressurized treatment unit was also investigated. The pressurized treatment unit has demonstrated the capability of operating with a high level of carbonaceous oxidation and nitrification at organic loadings of up to 13 g BOD5/ m2 of disc area per day. In comparison with the identical reference unit operated at atmospheric pressure, the pressurized unit demonstrated slightly improved BODs and COD removal efficiencies, a greatly improved level of nitrification and a substantially lower sludge production. All these characteristics improved with increasing pressure.Of particular importance the sludge yield coefficient was always significantly lower for the pressurized unit than for the reference unit and results such as 0.12 kg dry solids/ kg BOD5 removed at 6 bar pressure are highly signif1cant with regard to the requirements of the modern wastewater treatment industry. Additional investigations were carried out to determine the cost of a proposed full-scale pressurized treatment unit with a design based upon the findings of this investigation. These were compared with the costs of a conventional biological treatment process capable of treating an equivalent wastewater loading. The comparison between the pressurized unit and the selected processes were made for three populations (500, 1,000 and 3,000 persons). The sludge disposal costs of the pressurized unit were appreciably lower than those for the other processes. The results indicated that the cost of the pressurized unit (present value for a twenty-year period) and the costs of activated sludge and conventional RBC processes were found to be similar for the smaller populations. However a substantial saving could be obtained with the pressurized unit for the larger populations. In addition there is an indication that the land requirement of the pressurized treatment unit decreases appreciably as the flow rate increases.