Grey-water is any wastewater except water from toilets (which is called blackwater). It is estimated that 70-80 % of the waste water coming from a household is greywater.
Conventional plumbing systems dispose of greywater via underground septic tanks or sewers which often get overloaded, are difficult to maintain and have poor treatment, spewing it into lakes and rivers.
For all the planning of a modern world, we are still throwing our waste in our water sources.
Grey water is rich with nutrients (mainly nitrogen and phosphorus), bacteria and toxins depending on what the water was used for. When this water is expelled into fresh water bodies, these excess nutrients cause rapid algal growth (algal bloom). When these algae die, numerous bacteria and fungi feed on them, taking excess of oxygen from the water and smothering the aquatic life within (eutrophication).
High levels of nitrogen in the form of ammonia (NH4+) can also extract oxygen from the water to form nitrates (NO3-). Toxins too affect the aquatic life and enter the water cycle polluting the entire entity of water.
Thus, it is important to reduce both the nutrient levels and the toxins level of the waste water before it is disposed.
The most effective way of disassociating nutrients from the greywater is to allow the microbes in the system or biologically active soil to devour the organic matter in the water. The nutrients released by these microbes can be used for agriculture, forest and landscape irrigation while the water either infiltrates the soil or is re-used.
One important aspect of water conservation is allowing water to infiltrate into the ground wherever possible. When water passes through the soil, it cleanses itself and recharges the ground water table. We should, however, be careful of the quality of the water going down the soil, or we risk polluting our groundwater reserves.
The existing farmhouse has a single kitchen which generates about 100-120 litres of greywater everyday. We had initially put this water around a group of banana trees (old design here), but due to slow drainage in the soil, there was always some water that was either standing in the trench or had to be channelised to flow through the garden. A more effective and less demanding system was needed to deal with the greywater and make optimum use of its nutrients.
- it was important that the greywater doesn’t flow into existing drains and swales to prevent contamination of water bodies.
- We had initially installed a grease trap in the system. However, a grease trap can really be a trap. It involves regular maintenance (every week in this case). Since there wasnt any grease disposal system from the grease trap, this grease would end up in the compost, which got us wondering why do we even need to separate the grease in the first place.
- A lot of greywater designs suggest use of perforated pipes for distribution of water across the length of the pipe. In our experience, they clog pretty often – from grey water sedimentation, earth, mulch etc and we wanted to avoid this in our solution.
We wanted to keep the system simple and working at all times, so that users are encouraged to participate rather than dismiss the system.
A little more about grease traps
Grease and oil cause clogging in sewer pipes by accumulating on the inside. Over time, these deposits get larger and reduce he sewer pipe capacities and lead to overflows (potential cause of diseases). The cleaning of these deposits is not easy and can be very toxic. So a grease trap is essential in homes cooking with lot of oil and meet, and, of course, in restaurants.
Greywater mulch pits are an easy to build and maintain solution for re-using greywater in a garden. Mulch pits are really a hole in the ground filled with mulch where the greywater can be directed from a kitchen or other source. The nutrient rich water provides food for the microbial life feeding on the mulch and the solid food wastes carried along with the greywater. The water loses part of its nutrients as it passes the mulch and finally infiltrates the soil. Thus mulch pits help in filtering water before it soaks into the ground.
Mulch pits can either be dug close to trees so that the nutrients released from the water and decomposing organic matter are available to the plant (along with the water going down the soil), or the mulch can be allowed to decompose completely in the pit and be used as compost. This compost can again feed trees or vegetable gardens (preferably not for root crops).
Mulch pits also prevent evaporation of the water, keeping it in the system for longer and thereby helping in restoring its quality.
Based on the daily greywater generation and drainage of the soil, multiple mulch pits can be dug for a system to prevent water logging in a single pit.
We decided to build two mulch pits for the kitchen wastewater around a pair of banana trees that already existed. When one pit gets full, we shift the water to the other pit and let the first pit sit for a while to let the excess water seep down and the mulch and organic matter to decompose. The idea is to use one mulch pit for about 4-6 months and then change to another for about the same time. The durations can be adjusted as per the need and state of the pit.
Also banana is a heavy feeder and soaks in a lot of water and nutrients. Thus, the mulch pit in this case will serve two functions:
- Generate compost
- Generate biomass
The volume of each pit (160 litres) is about 4 times the peak flow (40 litres) that leaves the kitchen wash area at any given time. This is to allow for the space taken up by the mulch, food waste and a bit extra. The mulch pits are about 100 cm deep. The increased depth was to account for the volume as we could not increase the surface area due to space constraints.
The main outlet pipe is connected to a flexible hose that can shifted to either of the mulch pits.
For grey water systems, pipes of diameter 1.5” to 2” are recommended. Any smaller and clogs are more likely. Any larger and solids might stick on the bottom of the pipe. The outlet pipe already in use in the kitchen is 2.5”. Because of the short distance between the source and the outlet and a fall of about 10 feet (3 metres), we decided not to change the pipe. The fall will keep the greywater flow gravity fed.
Where the pipe is horizontal, it slopes down by about 2%. At this slope, the water and solids move more or less at the same speed. If the slope is lesser, the water doesn’t flow well, and at greater slopes, the solid waste lags behind the flowing water, building up over time and clogging the pipes.
We also introduced worms from our compost piles to the mulch pit. These worms will help in tunnelling through the mulch and grey water residue deposition, creating more air spaces and favouring aerobic decomposition. The worms also have the ability to kill some pathogenic micro-organisms.
Greywater systems also require a change in behaviour. We have been using mild, natural substitutes to the easily available, cheaper and stronger soaps and cleaners.
This system is in operation since a month now and it works smoothly without clogging or other hassles. Besides, the banana trees seem to be enjoying the feast.
The performance of the mulch pits has to be observed in monsoon and cold season (October- January). Too much water from the rain could leave the soil saturated and the infiltration could be slow. Also low temperatures could decrease the microbial activity taking longer for the mulch and other organic waste in the pit to decompose.
Create an Oasis with Greywater by Art Ludwig