A completely mixed anaerobic filter was found to effectively remove organic matter concentrations in high-strength solid waste leachate over a range of organic loadings and shockloads. Recirculation eliminated the need for buffer solutions. Testing of a fixed film biological reactor model showed that the substrate removal rate is primarily affected by substrate concentration, specific surface area, flow rate, and temperature of the unit. Studies of the biological aerated lagoon or extended aeration process were conducted in six completely mixed reactors (no recycle) fed with undiluted leachate. Phosphate requirements of the aerobic biomass were extensively evaluated. Kinetic constants were calculated for optimum conditions. The settling and dewatering characteristics of the sludge from the aerated lagoons were studied. The combined treatment of leachate and municipal sewage in a conventional plugflow activated sludge unit was found to effectively treat high strength leachate. The test unit was not able to treat the high strength leachate at = or > 4% of the municipal sewage flow rate. Physical-chemical treatment methods are not effective in removing large quantities of organics from the leachate and that biological pretreatment is required; these methods were therefore tested using aerated lagoon effluents. Ozonation, activated carbon, anion exchange resins, and reverse osmosis were studied. Treatment costs were estimated for leachate flows of 7.6 and 76 l/min containing 5000 and 25,000 mg/l BOD5.