TED- Technologies for Economic Development, a Non-Governmental Organisation (NGO) based in Lesotho, is constructing biogas systems since 2004, and since 2006 in combination with more components as DEWATS (Decentralised Wastewater Treatment System) plants.
The function of such a plant is to treat waste water of mostly private households in different steps: A digester, which is working as an airtight settler where produced biogas is stored; an Anaerobic Baffled Reactor (ABR), where organic pollution in waste water is reduced biologically in several separated chambers; an Anaerobic Filter (AF), where layers of micro-organism on filter material treat the sewage further and a horizontal Planted Gravel Filter (PGF), where the roots of aquatic plants reduce the phosphate load as a post treatment.
Biogas/DEWATS technology and processes depend on micro-organisms, which are especially sensitive to temperature changes. To obtain more insights into the development of treatment processes during the winter months, TED started a measurement campaign in September 2010, at the end of winter, involving the two installations at Children’s Home and St. Angela Chesire Home. Parameters such as COD and temperatures were measured. In June 2011 – in mid-winter, further samples have been taken at the Children’s Home Biogas/DEWATS plant.
In Children’s Home a plant for 45 people was build. A biogas digester of 9m3, a 7 chamber ABR, a 3-chamber AF and a PGF treat the waste water of this institution. Afterwards it can be used in the garden as irrigation water and fertilizer.
On a representative day, which means no strong rain or other unusual incidents, samples at different points of the plant, for example the digester, ABR and PGF outlet were taken. Alkalinity, pH and system temperature were measured on site – accompanied by the actively interested children living in this home.
The pH value shows in which environment the micro-organisms have to work. For methane bacteria the best pH ranges between 6.5 and 8.1. Then proper biogas production can be expected. Also the system temperature is important for the operation. Micro-organisms are adapted to special temperature ranges. Already small fluctuations can stop biogas production and degradation. In general the lower the system temperature is the lower is the operation of the micro-organisms. The alkalinity gives information about the buffering capacity of the water. It says how much acid can be added until the pH reaches 4.3.
For the laboratory samples have been taken to measure the settleable solids and the COD.
In every treatment step of the system organic matter is degraded. The COD parameter tells how much oxidable matter is contained in the wastewater, and at the same time the load of pollution.
As the digester and also the ABR are working as settler, measuring the settleable solids at different points of the system results in an overview of their performance. After each treatment step the amount of settleable solids should decrease.
The following diagram shows the COD removal in the system.
Filtered (red) and total COD (yellow) were measured at the outlets of the digester, ABR, AF and PGF. Both the filtered and the total COD decrease continuously with every step. In the PGF there are no more settleable solids (filtered and total COD are at the same value). Up to this point solids were either biologically degraded or settle in the digester, ABR or AF.
The PGF effluent quality is within the range of the wastewater standards established by the German Government , which limits the COD for percolated water below 200 mg/l. The effluent COD is 111 mg/l. This low value could be explained with the few residents living in the home during the time of measurements. Although the system temperature is relatively low (12 °C) the degradation works satisfactorily.
To obtain more specific results, more measurements during different season should be carried out.
More information about the measurements in Children’s Home can be found in the detailed report.