Martha W. Gilliland

Land application of domestic wastewater, wood production, and sludge composting: An integrated design

Abstract Regulations set forth by the United States Environmental Protection Agency encourage the use of innovative domestic wastewater and sludge treatment and disposal techniques. A process which combines land treatment of wastewater following primary sedimentation with tree production and sludge composting was evaluated for land requirements. The sludge composting process generally utilizes a bulking material to reduce the moisture content and increase the carbon/nitrogen ratio of the sludge. The cost of the bulking material, which is often woodchips, can represent a substantial portion of the total system operating cost. Growing trees in the land application system produces both woodchips for composting, and firewood, for which there may be a market. Background information, including design criteria, is given for the land application and composting processes. In addition, laws and regulations pertaining to both processes are also reviewed. The general range of land area necessary to apply a wastewater flow of 0.044 m3/s was determined for the Omaha, Nebraska area. The analysis included the land requirements based on hydraulic loading (dependent on soil permeability) and nitrogen loading (dependent on nitrogen uptake by the crop). Calculations showed that 25–320 hectares would be required according to the hydraulic loading criteria while 41–85 hectares would be required according to nitrogen-loading constraints. Only 2.2–11.2 hectares would be required to grow enough trees in order to produce woodchips for the composting process. Thus, the majority of land under irrigation could be used for the production of firewood.

Land application of domestic wastewater, wood production, and sludge composting: an economic and energy production analysis.

Abstract An innovative wastewater treatment system was analyzed for system costs, energy requirements, and energy production. The system integrates the land application of wastewater on poplar trees with composting of the primary sludge using woodchips from the poplars. System outputs, from which revenue can be generated, include firewood and compost. Costs and energy use were compared with those associated with a traditional system using trickling filters and land disposal of sludge. Additionally, the sensitivity of costs to the discount rate, to revenues from the compost and firewood, and to land prices was evaluated. Results indicated that the innovative system could be economically competitive with the conventional system if some revenues were obtained from the firewood and compost. And, energy use by the innovative system was 35% less than for the conventional system. Comparative costs were most sensitive to land prices and compost revenues, less sensitive to firewood revenues, and showed almost no sensitivity to the discount rate.