New life for sewage sludge

Wednesday, 6 May, 2015
Sewage may not look like a particularly precious resource, but it is home to phosphorus and other products, such as metals and biogas. EU-funded researchers are developing and demonstrating more efficient ways to extract these, turning waste into a valuable and renewable resource, creating new opportunities for companies and reducing its impact on the environment.

One of the research and demonstration projects is P-REX, which is helping to develop a market for recovered phosphorus from municipal sewage. Phosphorus, which is excreted in urine, is a nutrient needed by all life.

Plants extract phosphorus from soil; farmers replenish it via fertilisers. To make the fertilisers and animal feed, the EU depends on imports of phosphorus. About 90% of the demand, some 975 000 tonnes annually, is imported mainly from North Africa and the Middle East, where it is mined from phosphorus rock.

This dependence could be reduced by recycling phosphorus from sewage sludge, the thick semi-solid material left over after treating municipal wastewater. Recovered phosphorus from this sludge could theoretically cover about 20% of Europe’s current demand, says project coordinator Christian Kabbe of Kompetenzzentrum Wasser Berlin in Germany.

P-REX, which ends in August 2015, is advancing that process by evaluating the costs and benefits of 10 currently promising technologies to recover phosphorus from sludge, or from the ashes left over from its incineration.

Alongside market analysis, P-REX will produce a guidance document for policymakers and industry, outlining the suitable phosphorus recovery options and recommendations for fostering a European market for products containing recovered phosphorus, such as fertiliser.

In 2010, some 42% of Europe’s municipal sewage sludge was treated and used on farmland, 27% was incinerated, 14% was disposed of by landfilling and about 17% was disposed of in other ways, according to Eurostat.

But there are wide variations across Europe – and even between regions – in how each country currently treats and disposes sludge. The P-REX recommendations are tailored to these differences.

“The aim is not to change the modes, but recommend suitable recovery technology based on the infrastructure already in place,” Kabbe explains.

In Germany, for example, more than half of all sludge is incinerated, with the rest applied directly to land, says Kabbe. In the south, where most is incinerated, recovery from ash makes more sense. In the north, treated sludge is usually applied to agricultural land directly, so alternative technologies to recover phosphorus would be more appropriate.

A market for recovered nutrients

But there will be no recycling without a market, which is dependent on price, quantity, handling, distribution, and the operational benefits of recovering phosphorus from sludge, says Kabbe.

“We know a lot about the technologies and how recycling phosphorus could work, but now we need to do the obvious and take action,” he adds. “Industry needs incentives, such as reasonable subsidies and EU-wide policies, to reach the economies of scale needed to reduce our dependence on imports.”

Currently, only about 2 000 to 3 000 tonnes of struvite, a phosphorus-rich mineral, is produced each year in Europe from municipal sewage, says Kabbe. It represents the lowest hanging fruit and to increase this, the project launched an online platform in January 2015 to link European suppliers of recovered phosphorus with potential buyers.

The results of the project will be disseminated at international workshops and regional events to encourage more production.

“P-REX will provide an essential milestone for our future development into a recycling society,” says Kabbe.

Efficient processes

The EU-funded projects RecoPhos, ROUTES, and END-O-SLUDGE are also investigating the recycling of sewage sludge.

RecoPhos, which ended in February 2015, developed an innovative thermal reactor that breaks down sewage sludge ash to obtain pure phosphorus for industry. Other by-products include silicate slag for use as a binder by the cement industry, syngas for energy generation, and iron slag for use by steel producers.

The project scaled up the technology from the lab to a bench-scale reactor able to process several kilograms of ash an hour.

Meanwhile ROUTES identified more efficient techniques to make sludge safer and to improve quality so that it may be used directly on agricultural land as a liquid fertiliser. The process centres on thermal pre-treatment at 135°C, followed by digestion by heat-loving organisms. This kills harmful contaminants, and also allows the recovery of valuable resources for industrial use, such as methane, ammonium sulphate and biopolymers.

The project ended in April 2014 after conducting tests on the various techniques and technologies in the laboratory, at pilot scale and at full-scale at three different plants in Italy and Switzerland.

Integrating technologies

END-O-SLUDG, another complementary project, demonstrated how a toolkit of breakthrough technologies and processes could be used to improve the efficiency of traditional sludge treatment methods. The toolkit includes information on the use of different types of coagulants and other agents, plus ultrasound and micro-milling techniques, to treat sludge and make it safe for use as fertiliser.

The project also designed in detail a pilot granulator and an alternative method that uses a water-source heat pump to dry out sludge.

The researchers showed that the techniques could cut initial sludge volumes by half, allowing energy savings in further steps along the treatment chain. They also demonstrated how techniques using ultrasonic waves and micro-milling could raise biogas extraction to 80% of what is available from sludge, compared to 45% using traditional treatment methods.

END-O-SLUDG pioneered the use of probiotic bacteria in treated sludge to eliminate the harmful E. coli bacteria that often re-grow after initial treatment. The project also reported that it made significant progress in turning sludge into safe, high-quality, easily transportable and spreadable organo-mineral fertilisers.

Pilot trials on farmland indicated that the resulting product was at least equal to conventional mineral fertilisers used on crops, the project reported.

By the time the project ended in December 2013, it had produced data showing the fertilisers pose no additional environmental concerns or risks to human health. The results are also input for decision makers on whether to declassify products from municipal sewage as waste material, a step needed to open up the market.

“Declassification would represent a crowning achievement for the END-O-SLUDG project – completing the final step in the process from sludge volume reduction through to the transformation of the sludge that remains into viable, marketable and safe end-products that meet stringent agricultural and environmental standards,” states a project report.

Recycling animal waste

Meanwhile the BioEcoSim and ManureEcoMine projects are extracting products from livestock manure. Europe’s farm pigs and cows produce about 1.27 billion tonnes of manure per year, a largely unexploited resource of organic carbon and nutrients.

BioEcoSim, which ends in September 2016, aims to develop and demonstrate a pilot plant for the conversion of livestock manure into safe and stable fertiliser that is easy to handle, transport and apply.

The new process is a potential solution to the problem of surplus farm animal manure in regions with high livestock densities, says project coordinator Sukhanes Laopeamthong of the Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung in Germany.

Part of the energy required by the new process would be generated through the combustion of synthetic gas derived from the manure itself, a way to cut back on fossil fuels, he adds.

“Water reclaimed from manure will be utilised for livestock production and irrigation, while resulting soil-improving materials will be evaluated in agronomic trials with three representative plant species and three different soil types,” Laopeamthong explains. “This will benefit livestock farmers, allowing them to generate income from the sale of soil-improving products and electricity generated from synthetic gas, instead of paying the high costs for disposing of manure.”

ManureEcoMine, which ends in October 2016, also sees potential in manure. The team is developing an energy-efficient approach to the treatment and reuse of farm animal waste. The researchers are constructing and validating a pilot plant to demonstrate an integrated thermal process (superheated steam dryer and pyrolysis reactor) to treat raw pig manure.

The plant will demonstrate a phosphorus precipitation process to recover nitrogen, phosphorus and other minerals from pig manure. A separate unit will be used to recover ammonia. The eventual pilot unit will be able to treat 100 kg of raw pig manure an hour.

The project is also studying the impact of the resulting fertiliser on plant growth and soil health. Life cycle analyses of economic viability will help determine the sustainability of the concept, identify the most environmentally friendly technology and the most effective and safe strategy for reusing products extracted from sludge.

Sustainable sewage sludge management fostering phosphorus recovery and energy efficiency
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