NOVELTY - A compact wastewater treatment plant comprises at least three bioreactors in series containing rotating biological contactor between two microbubble/settling chambers. The bioreactor or rotating biological contactor includes discs for rotation about a shaft. Each microbubble/settling chamber has a settling chamber to collect suspended solids and a microbubble chamber with diffusers for injecting microbubbles into wastewater.    USE - For treating wastewater (claimed) from food processing industries (e.g. wineries, poultry, pig and cattle farms, fruit processing, fish processing plants and cheese plants); in municipal and domestic wastewaters (from e.g. subdivisions, hotels and resorts); and landfill leachate and bottoms from septic tanks with chemical contaminants difficult to biodegrade.    ADVANTAGE - The plant treats wastewater with high organic contamination and low biodegradability. The treatment unit provides high strength; and uses commercial microorganisms that enhances microbial growth in the settling chamber or promotes specific microbial processes. The configuration of the treatment plant permits more intense removal of pollutants and thus achieving effluent quality comparable to that of municipal plants. The plant removes total suspended solids, biological oxygen demand (BOD) and smaller amounts of organic nitrogen and phosphorus compounds. The cells of hexagonal cross section expose the greatest amount of surface area of the disc-forming material to wastewater and air during its rotation, thereby increasing the amount of biomass that can be formed on the disc and improving the performance of plant. The design of the discs prevents formation of septic conditions on the discs and therefore avoids offensive odors and promotes high rates of pollution removal. The structure of plant avoids wrapping of the discs and flat thus making the plant more efficient, with longer life expectancy, and thereby reducing costs. The contactor provides high efficiency converting ammonia (nitrification). Thus, because of the flow configuration of contactor, the surface geometry and interfacial area of the discs and the BOD removal efficiency is comparable to that of centralized municipal plants. The interlocking mechanism maintains a permanent air gap between neighboring discs, which promotes high aeration efficiency. The plant provides solids and sludge collection, high organic load removal, high biological phosphorus removal, and high organic nitrogen removal in a reduced construction space and/or high performance. The plant has a capacity to treat both high-strength industrial wastewater and municipal wastewater without recirculation; exhibits excellent removal efficiency and short hydraulic residence times; is small, compact, modular and cost effective; does not waste prime land. The plant performs continuous treatment; has low maintenance level; requires less skilled labour and control systems than activated sludge processes; does not produce undesirable odor; reduces production of sludge; is more economic than aeration ponds, septic tanks plus leaching beds, sequencing batch reactors, and other activated sludge plants. The treatment unit can treat raw wastewater with a pH of 6.3 plus minus 0.7 and produces an effluent of 8.3 plus minus 0.3, 95% of the time. The treatment unit exhibits treatment capacity of 3 (preferably 50) m3/day. The contactor treats on a continuous basis high strength wastewater of 300 - 12000 mg/l of BOD with average removal efficiency of 96%. The contactor treats municipal wastewater mixed with landfill leachate with 90% of BOD removal efficiency, and ammonia removal exceeds 95%. By adding a polishing stage like an approved biofilter, and chemical coagulants the unit can deliver tertiary effluent quality for discharge to surface water sources. The treatment unit connected to a properly approved biofilter, and or ultraviolet disinfection can meet the regulatory requirements for the treatment of wastewater from domestic wastewater and food processors like wineries.    DETAILED DESCRIPTION - A compact wastewater treatment plant comprises at least three bioreactors in series. The first bioreactor or first microbubble/settling chamber has a settling chamber to collect suspended solids and a microbubble chamber with diffusers for injecting microbubbles into wastewater. The second bioreactor or rotating biological contactor includes discs arranged for rotation about a shaft and is in fluid connection with the first microbubble chamber. The third bioreactor or second microbubble/settling chamber has a settling chamber to collect suspended solids and diffusers for injecting microbubbles in the wastewater and is in fluid connection with the biological contactor. The disc includes geometric cells having hexagonal shape (22A) or octagonal shape. The discs are interlocked such that wrapping of the discs is prevented. The shaft is enclosed within a sleeve, which prevents corrosion due to exposure of the shaft to the wastewater.    An INDEPENDENT CLAIM is included for treating wastewater involving: flowing wastewater into the first microbubble chamber for injecting microbubbles into the wastewater to promote oxygenation of the wastewater; flowing the wastewater from the first chamber through rotating discs having a biofilm of microorganisms; flowing the wastewater into the second microbubble chamber for injecting microbubbles into the wastewater; and recovering the treated wastewater.    DESCRIPTION OF DRAWING(S) - The figure shows a front side view of the disc of the rotating biological contactor.    hexagon cells of cells (22A)    passages (22C)    square shaped cells of disc. (23)