As Oman is an arid land facing water scarcity, any type of water conservation system is important for the nation. Greywater, or waste water generated from wash hand basins, showers and baths, accounts for more than 50 per cent of total wastewater flow to domestic sewers in Oman. A group of scientists at Sultan Qaboos University, through a study, proves that household greywater, after minimal and low cost physical treatment, can be reused for irrigating kitchen garden consisting of plants such as tomatoes, carrots, eggplants, grapes, other fruits and even date palm trees.
Dr Abdulrahim al Ismaili (pictured) , an assistant professor in Agricultural Engineering at Sultan Qaboos University, who lives in Al Khoudh 6, an area where currently no pipe water supply is provided, selected his own household to install a grey water treatment system. “Any type of water conservation or reuse system is vital for me, as I pay around RO 40 per month on water delivered in tanker trucks” he said. At his household, greywater, ie, used water from hand wash basins, showers, baths and discharge from laundry, dishwaters and kitchen sinks is eliminated and collected in a separate underground tank whereas the black water or sewage is disposed separately.
The greywater collected in the tank is then pumped to a treatment tank filled with layers of sand from sand dunes at the top, fine gravel at the middle, and then coarse gravel at the bottom. Above the sand, the water is filtered to remove large particles, hair, lint etc from the greywater. The water which undergoes physical treatment through the tank, then passes through a small chamber where chlorine tablets are kept for disinfection process. Now the treatment process is over and the recycled water is ready for irrigating the house garden and other plants. Dr Abdulrahim applies the treated greywater directly to the soil, not through any method that would allow contact with the above-ground portion of the plants. He irrigates mainly tomato, carrots and eggplants growing in his house garden.
According to Dr Abdulrahim, though the used water may contain grease, food particles, hair and any number of other impurities, it may still be suitable for reuse. “Reusing greywater serves two purposes: it reduces the amount of freshwater needed to supply the household, and reduces the amount of wastewater entering sewer or septic systems”. When asked why only greywater (sullage) and not blackwater (sewage) is recycled, he said: “Most greywater is easier to treat and recycle than blackwater, because of lower levels of contaminants. If collected using a separate plumbing system from blackwater, domestic greywater can be recycled directly within the home, and reused for irrigation. Recycled greywater of this kind is never safe to drink, but a number of stages of filtration and microbial digestion can be used to provide water for washing or flushing toilets. Fruit trees and vegetable plants grow well with greywater”, he said.
From the tomato plants grown in the backyard, Dr Abdulrahim collects at around 1kg of fresh ripened tomatoes per day. “Eggplant and carrot plants also give good yields. Now I am irrigating some fruit trees such as fig, mulberry and grapes in addition to five newly planted date palm saplings”, he said. At present, as part of a study, the house garden has been divided into three parts based on the type of water and nutrients provided to the plants. In the first part, the plants are irrigated with culinary water in addition to regular nutrients such as nitrogen, calcium and potassium. In the second compartment, the plants are irrigated with treated greywater and not provided with any extra nutrients other than those available in the soil. The third section is irrigated with treated greywater and provided with extra nutrients. “Apparently, the first section has the maximum yield which indicates that the soap, shampoo and conditioners traces present in the greywater are slightly affecting the production and growth of the plants”, Dr Abdulrahim said.
“However, if extra nutrients are provided, the production increases. I am very much satisfied with the efficiency of the greywater treatment facility installed in my household and I believe that this model can be emulated in households across the Sultanate which can substantially lower household water bills and save the planet in the long run. In almost every Omani household, there are big lawns and green yards which are often irrigated with home water supply, which leads to heavy water bills and draining of water resources. Installation of household treatment system at each household is a promising solution for the country. The cost of setting up a system is very cheap compared to the good results in the long run, Dr Abdulrahim said. In his experience, the system which has a capacity of 2 cubic meters of water, produces more treated water than what is required for irrigation. In fact, the excess treated water can be sent to toilet flush-tanks using small pumps. This may again reduce the consumption of supplied water.
This research project is supported by an internal research grant from the Centre for Environmental Studies & Research (CESAR) at Sultan Qaboos University. The project titled “development of low-cost and decentralised greywater treatment systems for handling, treatment and reuse in Oman”, aims at developing a greywater treatment system. The project is headed by Dr Mushtaque Ahmed and the team includes Dr Ahmed al Busaidy, Dr Abdulrahim al Ismaili and Seif al Adawi, all from the Department of Soils, Water & Agricultural Engineering of the College of Agricultural & Marine Sciences.