Algae are used to depollute sewage and produce compost

Brazilian and Dutch researchers use single-cell algae to try to solve the problem of managing the waste generated by sewage treatment.

By Reinaldo José Lopes  |  Agência FAPESP – A partnership between Brazilian and Dutch researchers has demonstrated that it is possible to convert black water – the “heaviest” fraction of household waste consisting mainly of a relatively undiluted mixture of feces and urine from toilet flushing – into a sort of alga farm.

Single-cell algae of the genus Chlorella feed on nutrients in this effluent. As they grow, they help to remove the pollutants it contains and simultaneously produce large amounts of biomass for uses such as composting.

The current research results were presented at a workshop held in early September at the University of São Paulo’s São Carlos Engineering School (EESC-USP), with researchers from both sides of the Atlantic in attendance. On the Brazilian side, the partnership receives funding from FAPESP, while European funding comes from the Netherlands Organization for Scientific Research (NWO). The four-year collaborative project is scheduled to end in January 2018.

According to Luiz Antonio Daniel, a professor in EESC-USP’s Department of Hydraulics & Sanitation and one of the coordinators of the partnership, its main objective is to solve the problem of managing the waste generated by today’s sewage treatment processes.

The main components of the feces and urine flushed from toilets include the carbon in organic matter, nitrogen, and phosphorus. When large amounts of waste are dumped in watercourses, nitrogen and phosphorus cause eutrophication – the excessive growth of aquatic microorganisms, especially algae – leading to potentially serious imbalances in aquatic communities and releasing pathogens into the environment. 

“In the sewage treatment processes most commonly used today, chemicals remove phosphorus from the water, and what’s left is a sludge that can’t be used for much else. Some Brazilian states ban its use as fertilizer in agriculture, for example” Daniel explained. “The sludge ends up being discharged into landfills. The cost of disposal is therefore considerable.”

Back to the closed cycle

However, the current disposal strategies were not always used. In her presentation to the workshop, Dutch scientist Grietje Zeeman, Emeritus Professor in Wageningen University’s Department of Environmental Technology, showed photographs of the system used to collect human waste in barrels in the Netherlands in the nineteenth and twentieth centuries, and the system was deactivated only in the early 1980s.

Fecal matter and urine collected from households were used as fertilizer by Dutch farmers back then. “With the system we have today, which can be called ‘flush and forget’, this nutrient reuse cycle has been lost. Our idea is to close the cycle again,” Zeeman said.

To attain this goal, the first step entails decentralizing sewage collection to prevent the dilution of black water and the nutrients in feces and urine.

“It wouldn’t be necessary to decentralize excessively. We needn’t treat the sewage of each home or building separately. We could have units serving a few thousand inhabitants, up to some 10,000,” Daniel estimated.

“Because approximately 50% of Brazilian towns have fewer than 10,000 inhabitants and only 25% have sewage treatment systems, many localities can introduce such systems from scratch.”

In the reactors tested by the team, microalgae of the species Chlorella sorokiniana use nitrogen and phosphorus from black water and the micronutrients present in human waste to multiply. The next step is collecting the layers of microbes that grow in the liquid. This can be done in two ways, according to Daniel.

“In the Netherlands, they mostly use sedimentation, in which a polymer is used to settle the algae at the bottom of the reactor, and they can be collected from there,” he said. “Here, we use flotation. We inject compressed air into the liquid, and bubbles form on the surface containing the algae as they float up. A mechanical scraper then collects this biomass and discharges it into a channel.”

Specifically, because they absorb the nitrogen and phosphorus in black water, the algae are rich in these elements, which are essential to the industrial-scale fertilizers applied today. To leverage this potential, it is necessary to use efficient biomass drying methods, according to Daniel. If algae are stored without being dried, their cells can rupture, and the nutrients to be used at the end of the process can leak out.

The partnership with the Dutch team, he added, has been very useful for purposes of comparison. Considering the differences between the two countries in terms of climate, it is possible to develop methods to optimize the production of algae depending on the context.

“The sun doesn’t shine year round in the Netherlands as it does here, for example, nor do they have our high temperatures, which sometimes inhibit algal growth. Therefore, the Dutch reactor we’ve tested at USP gets too hot. To achieve a larger scale, we’ll have to make several adjustments,” Daniel said.

Optimizing the process so that it works on an industrial scale is the next step in the studies. Field tests will be conducted at the Monjolinho sewage treatment plant in São Carlos.

An advantage of using Chlorella in the process is that these algae are already present in nature and do not require genetic modification to do their work. Hence, there should be no problems with regard to the disposal of treated sewage into rivers and lakes.

“If you leave a sample of sewage out in the open, it will naturally be colonized and go green,” Daniel said, stressing the importance of seeing black water and other effluents as potential resources.