C.A.R.T. - Centre for Appropriate Rural Technology

Waste Management

One of the most common problems facing rural communities around the world is that of waste disposal. The incorrect disposal of animal and human waste can lead to the poisoning of waterways, render land un-farmable and cause numerous fatal diseases such as Cholera, Typhoid and Diphtheria. CART needed a way to not only dispose of its waste but also to find a way of actually making this waste useful. After much research it was decided that a Bio-Digester could provide the solution to this problem, however no design existed that could be cheaply replicated in rural areas – so CART made one. Conventional Bio-digesters cost in the region of US$30,000, CART’s costs about US$500.

The simple design consists of a hole 7m x 1.5m x 1.5m into which a reinforced PVC cylindrical black bag is placed. At one end is the "inlet" situated at the bottom of the bag, a pipe from the toilet block brings all human waste into the bag together with a hatch for animal and kitchen waste. Two outlet pipes are situated at the other end of the bag, one at the top for gas removal the other is 1/3 of the way up for liquid removal. The design incorporates a black colour so as to attract solar heat to accelerate decomposition. The placement of the outlet pipe is important as it is necessary to maintain 1/3 liquids : 2/3 gas.

Due to the sealed nature of the bag all this waste begins to decompose anaerobically releasing methane gas, water and sludge waste whilst also killing harmful pathogens such as E-Coli. The methane gas is released and captured using the valve at the top of the outlet end of the bag, this gas can be used for cooking, lighting and heating (although CART currently intend to use the gas solely for cooking.) The water is released through the outlet pipe, this water is known as ‘High nutrient’ or ‘Black’ water. Less than 10% of the products are solids and therefore only need removing from the bag every few years, these solids can be used as a high nutrient fertiliser for fruit trees or for plants where the fertiliser does not come into direct contact with the crop. The black water is mixed with grey waters, such as shower or washing water (CART only uses organic, Bio-degradable washing products to ensure no toxins are introduced to the cycle.) This water is filtered through sand and charcoal and used to irrigate the food gardens. Excess water, together with runoff from the gardens is collected in the Dam, from which it can be used to water gardens in the dry season or filtered further making it safe for drinking (See water cycle)