EcoHouse Research

Maxine  Jordan (MEng, 2012)

 

Driven by the quest for affordable, sustainable, low-technology wastewater treatment options for developing cities, this report is a study of the limits of the applicability of the anaerobic digestion of human waste. This will constitute one of the first research papers which will be fed into the Eco-House Initiative ’library’ of research; The Eco-House Initiative is a student-let organisation, based in the Cambridge University Engineering Department, which seeks to develop design systems and solutions to provide low-cost, sustainable, permanent housing in the developing world. A field trip with the Initiative to Quito, Ecuador, fueled a desire to find alternatives to conventional wastewater management practices for those who the municipal sewer system does not reach. Wastewater treatment is under strain in many of the rapidly growing, developing cities of the world, therefore more sustainable options must be sought as alternatives to connection to a centralised, and often aging system.

 

Anaerobic digestion is a technology which has the potential to deliver significant benefits: sanitation, fuel, and additional revenue. However, in order to assess at which scale, and under which conditions one or all of these benefits can truly be yielded, systematic and analytical modelling of this complex process is required. By modelling the process of anaerobic digestion from the basic kinetic theory, a method of assessing the performance of biogas recovery from human waste was developed, incorporating a set of variables which are said to define a particular scenario. The performance of such a system is assessed in terms of COD removal, net methane yield, and economic viability. A particular scenario is considered to be characterised by its ambient temperature, the volatile solids of the feedstock, daily water consumption per capita, number of people served, and the local economic conditions such as the value of fuel (in $/GJ) and construction costs. The operating point of maximum economic efficiency is defined as one which gives the minimum payback time, where the incurred lifetime costs of a project are associated with the cost of construction alone, and the annual ’income’ as the value of the displaced fuel.

 

By firstly considering the economic value of the gas as the only source of value, it was found that at any temperature within the range studied (20ºC to 60ºC), there exists a single set of operating conditions defining a point of maximum economic efficiency. These operating conditions are given as temperature, retention time, (and thus volume and cost) and give a particular methane yield. It was found that for operation at ambient temperature (no heating costs) this corresponding methane yield is constant for all temperatures, and around 50% of the ultimate methane yield. This value is too low to provide any use per person - therefore in order to provide useful per capita yields of methane, either community systems must be considered, where the yield may be distributed among fewer people than are feeding the digester, or livestock should be added. While a higher yield is physically achievable, this would incur a sacrifice of economic efficiency. The energy costs of heating the digester were found to be too high at low loadings for provision by the methane produced alone, therefore only digesters run at ambient temperature are considered.

 

It was found that at low water consumption, the effluent COD concentration of a digester will be much higher (3140mg/l) than the allowable discharge, but a consumption of just 60l pcd is enough to dilute it to national Ecuadorian standards of 500mg/l. Although not providing full pathogen kill, anaerobic digestion is considered an incremental increase in sanitation, which, with the added benefit of providing potential sources of fuel and income, makes it a form of wastewater treatment to consider.

 

Associated with anaerobic digestion are multiple benefits other than the production of methane - these may include but are not limited to the potential for revenue from carbon credits, opportunity costs associated with cooking with biomass, and its health issues, the potential revenue from the effluent fertiliser, and the effectiveness of COD removal. These additional sources of value can all be quantified where applicable, and be incorporated in the calculation for payback time, giving corrected operating conditions truly representative of the context, and a reduced minimum payback time.

 

It was found that the yield of methane per person is low when anaerobic digestion is run on human waste alone. However there are many other benefits arising from implementation of this technology which must be identified and enter the analysis, to identify the minimum payback time which could be expected, and enable rational and informed decisions regarding its implementation.