Strawberry Fields Eco Lodge, Ethiopia: Black Water Filtration System

Black water is the term applied to domestic waste water which carries solid organic waste materials and has a high level of nitrogen and phosphate containing compounds which may be in soluble and non soluble forms. Black water is generally assumed to refer to discharge from flush toilets, while grey water refers to outflow from showers, baths and hand basins, which contains no solid material and generally lower levels of nitrates and phosphates. Since all our toilets at Strawberry Fields are dry composting toilets our black water system does not have to deal with human waste at all. However, what we are dealing with is the waste water from the kitchen. Kitchen water can be considered as black water because dirty plates, frying pans and utensils carry a lot of fats, starches and protein. As well as this, the detergents which are necessary to remove all those from the surfaces of the utensils are stronger than body soap and carry a lot of phosphates which need some breaking down. The resulting mixture of soapy water, fat, protein and starch will quickly become very rancid if bacteria begin breeding in it, as it will go anaerobic and start producing swampy smelling gasses like nitrites which are poisonous to most plants (except for swamp plants which are adapted to deal with them).

How do we deal with this mixture then? The first stage is to remove as much of the solid material as possible. This is simply done with a fine mesh sieve. Dishes at Strawberry Fields Eco Lodge (SFEL) are washed in a basin made from the bottom ¼ of an oil drum. The resulting waste water is poured through the sieve into a normal kitchen sink. The outflow from the sink flows into an inverted 15l plastic food-oil container. The outflow from this is covered by a second sieve of smaller mesh. From there it flows down a 50mm PCV pipe, 5m to the beginning of the aeration cascade with all the large particle solid matter removed.

The aeration cascade is an underground stream down which the water flows between a series of little pools. It was constructed by digging a trench 1m wide, 12m long and with a depth grading from 50cm at the shallow end to 1.5m at the deep end. The bottom of the pit is the basic clay soil substrate of the SFEL site.

Stone blocks are dressed into slabs of approximately 30 – 40cm by 30cm by 5cm. The slabs are hand dressed from local basalt. They are buried 5cm into the clay substrate in two rows which run down the middle of the trench making an alley-way down which the water will flow. The Alley way is 20cm wide and there is another 30cm gap on each side of it. However we don’t want the water to just run down the alley way to the bottom. We want it to move as slowly as possible, so a series of smaller stone wedges are put across the alley way, driven into the clay to make barriers at 30cm intervals along the alley way, which are 5 to 10cm high. The clay substrate is then covered by 3cm of sand. The two gaps to either side of the alley-way are filled with gravel at the bottom, which is covered with sand. The sand is pushed into the gravel where if fills the gaps, which will stop clay from moving into the gravel. Next the tops of the stone block are tidied up with a bit of cement to make the top of the alley way level, and concrete slabs are put on top to cover the alley-way over. The slabs have 50mm pipe set into them at 3 points along the length of the cascade, which provides for aeration. Banana suckers are planted into the sand on either side of the alley way at 75cm intervals on alternate sides. The slab is then covered over with more gravel-sand mixture, then a layer of sand and then with lovely rich silt from the bottom of a pond which we are digging out for a second time. The solid material is removed from the sieve daily and composted along with our other kitchen waste. (More on this to come!)

The idea of the design is that the water discharging from the kitchen sink will enter the top of the cascade and be stopped at the first barrier where a pool will form. The water in the pool will present a large surface area to the air, so will be quite well aerated. As it sits there the water in the pool will drop its non dissolved matter which will form a scum on top of the sand that will quickly develop a bacterial film. Hopefully this will remain aerobic due to the large surface area and availability of oxygen in the air in the gap between the water and the concrete slab which forms the roof of the tunnel in which the cascade is now functioning. Water sitting in the pool will now either soak into the ground beneath the pool, or percolate out through the gaps between the rock slabs which make up the walls of the cascade. Here it will be sucked up by the banana trees as they grow. As more water comes down from the kitchen sink and the pool fills up, it will over-flow and spill into the next pool and so-on down to the bottom of the cascade. By the time it reaches the bottom of the cascade most of the organic compounds in the water should have been broken down by the bacterial film on the rock surfaces and the bottoms of the pools. This is of course highly experimental but we keep observing as we go! The remaining water that reaches the bottom of the cascade will have no choice but to soak into the ground or build up at the bottom of the trench. If the drainage of the soil is inadequate we may have to extend the trench further and use more sand and gravel in the bottom to improve the drainage. Hopefully the drainage is good enough and the banana trees are thirsty enough to keep the water from backing and flooding the cascade.

Another potential addition to the system would be a septic tank that could be put between the kitchen sink and the cascade/soak away. This would trap and remove all the solid material from the water and remove the need for sieving. That would be more labour efficient, but has its own disadvantages, the loss of composting material being one of them.