Dry urine has great benefits for farmers: Scientist explains reuse techniques

Human urine is a rich source of nutrients for plants but processing it can be complicated using existing sanitation systems. Drying diverted urine can be revolutionary in harnessing its benefits. Prithvi Simha, a postdoctoral researcher of environmental engineering, Swedish University of Agricultural Sciences talked to Down to Earth on how urine can be separated in designed toilets, collected and treated to develop fertilizer pellets. Simha wrote a report on the topic in Source magazine in February 2022

Shivangi Agarwal: How can urine drying prove to be beneficial?

 

Prithvi Simha: Urine is rich in potassium, nitrogen and phosphorus. When we flush, the urine moves through sewer lines to treatment plants and gets diluted with flush water. This complicates the process of nitrogen removal from the urine, which can be used for increasing the fertility of the soil. Urine can be treated at source by collecting it separately and drying.

The technology of drying is quite old as we have produced dried food and other materials. We already have the resources we need to create a urine-drying toilet. High end ceramic urine diverting toilet might not be feasible for an average-income household. However, a simple canister or a drum can be installed to collect urine to make it cost-effective.

To balance the pH-level of urine, stabilisation is done by adding limestone and making it more alkaline. It is then air blown at a certain temperature and humidity level to remove water and keep intact the nutrients in a non-volatile form. To prevent odour or urine from travelling back in the toilet bowl, a concealing trap — ‘S’ trap or ‘C’ trap — pipe can be installed.

SA: Do you think the solution is economic for rural India?

PS: Yes, it can be a solution for rural India. In rural India, to make the urine-drying system economical and user-friendly, multiple urine-handling chambers can be avoided and an economically scaled semi-centralised system can be installed, connecting several urinals / toilets together. 

Multiple urinals or urine-diverting chambers can be connected to a single container / chamber with a dryer to treat and dry urine. The income generated by selling the end product (fertiliser) can be well used for operation and maintenance of the system.

SA: Now that rural India has constructed their onsite sanitation systems, do you suggest retrofitting of toilets?

PS: The conventional toilets can be easily retrofitted without demolishing or installing new toilets using an insert. Inserts are both commercially available. The insert can be fitted in the toilet bowl to divert urine. 

The focus here is to have a treatment system afterwards. In a village with 20-25 households, a centralised container and blower can be installed. The operation and maintenance cost, hence, will be shared among the communities and the money generated from selling the fertiliser can further be divided.

SA: Do you think people will accept the use of these pellets? What can be done more to motivate people to adapt these technologies?

PS: We feel a lot of behavioural change is necessary. But in reality, the idea of safe urination and defecation is nothing different. For a urine-diverting toilet, it’s only convincing that needs to be done. The males need to sit down and pee unless they have a urinal installed. As an individual, I can set an example by using the toilet and motivate others to use it.

SA: What are some of the successfully implemented projects?

PS: Durban, a city in South Africa, has implemented 80,000 urine diversion toilets and 1,000 community sanitary blocks with male urinals.

In Malmo, the fastest growing city of Sweden, a urine-diverting toilet has been retrofitted with an on-site urine dryer. It has the capacity to dry 10 litres of urine per day and retains 90 per cent of the nitrogen present. A solid powder with 15 per cent nitrogen by composition gets generated at the end of 30 days, which can be converted into pellets to be used as fertiliser. 

SA: What are the benefits of the end products?

PS: Urea is rich in nitrogen and phosphates, which improves soil fertility. The fertiliser can be made into pellets by manipulating the nitrogen-phosphorus-potassium ratio as desired. For example, adding biochar to the fertiliser adds carbon to it and reusing struvite can add phosphates. 

The fertiliser is lab tested in three countries and field tested. It is best suitable for paddy crops like barley, wheat, maize and sugarcane. The food or liquids produced using the fertiliser is sent back to the markets, sold and consumed by households. This helps in building an urban-rural linkage as the more people urinate, the more we save our environment and wastewater treatment plants and reduce dependency on fresh water.