Greywater Towers

Greywater (the wastewater from bath, laundry and kitchen) accounts for around 50 to 80% of the residential wastewater with manageable contamination levels. It therefore offers great potential for reuse in agriculture, particularly in areas where water is scarce. Greywater towers are low-cost solutions that allow for the reuse of the water and nutrient content in greywater and productively using it for gardening purposes.

Greywater towers are cylindrical and made out of simple plastic bags or clothes, reinforced with wooden poles, filled with a soil mix (soil, ash and/or compost/vermicompost) and anchored into the soil. Vegetables are grown in holes cut in the sides of the bag. Each day, the available greywater is poured into the bag. The soapy water contains nutrient on which vegetables can grow successfully. The soapy water can be cleared out of the system by pouring two buckets of clean water into the column once a week.

For the construction of a greywater tower, a circle has to be marked out on the ground with a diameter of around 80 cm. The bottom layer of the tower has to be dug out and the wooden side poles need to be firmly planted into the ground. A shade cloth will be wrapped around the poles and the resulting cylinder will be filled with gravel in the middle and the soil mix all around. The backfill has to be well mixed before applying it. A bucket with its bottom removed can be placed at the bottom in the middle of the tower to increase stability and improve the flow regime. Small stones should be packed in the bucket to increase weight. To prevent fast flow of the water through the bucket and to achieve even water distribution, small stones (provided they are evenly packed) should be used. Once the bucket is filled up with stones, it is backfilled with the soil mixture. Then, the bucket can be removed (and reused), leaving the stones in position. The soil should be humid but not too wet, when packing the tower. The soil should be evenly distributed as compacted water will not let water flow through. The procedure of placing the bucket, filling it with stones, backfill it and then remove the bucket has to be repeated for each soil layer until the top layer of the greywater tower (around 1 m in height).

Greywater towers are ideal for leafy crops such as spinach. Ideally the planting holes should not be one above the other but staggered diagonally, providing more space for root development. Different crops which use the soil in a complementary way should be grown to facilitate biological control of disease and pests. Tomatoes and onions can be planted on the top of the tower. Garlic and onions are also useful to prevent plant diseases and pests. If crops require trellises this can be provided by extending the vertical uprights and joining them with wire or string.

One of the main advantages of the method is that little labour for construction and little operation and maintenance is required. Once people have become familiar with the towers, they are easily maintained. The towers can be positioned right at the back door so that it is easy to pour the wastewater into the tower.

It is difficult to predict how much water is required, and the necessary experience will usually come over time. In the beginning it is recommended to apply around 20 L per day and fine-tune it according to the exact water demand. If water forms a puddle around the bottom of the tower, too much water is being applied. A possible answer to this problem would be to make a second tower.