25 August 2015
From wastewater to fertiliser: current and future opportunities
A summary by Dr Jane O'Sullivan
Emma Thompson-Brewster is a PhD student in the nutrient recovery group, within the Advanced Water Management Centre. She gave us a fascinating overview of technological developments that may one day soon help to close the nutrient cycle, getting some of the nutrients we strip from soils (and phosphate mines) back to soils and new crops.
The work mainly pertains to domestic sewage, which is now usually treated through centralised plants using activated sludge – a system developed in 1913 involving aerobic, anaerobic and anoxic conditions. Aeration consumes up to half the energy used for sewage treatment. Waste sludge is often already used in fertiliser for non-food crops such as cotton. But phosphorus is the only nutrient efficiently recovered in sludge, and may be contaminated with unwanted persistent organic compounds and heavy metals. The nitrogen, in particular, is deliberately lost through a denitrification step, to minimise nutrient pollution of effluent water.
Emerging resource limits include potable water, mineral phosphorus deposits, and the energy requirements for nitrogen fixation – which uses around 1.5% of global energy consumption. These motivate a paradigm shift from water treatment for safe discharge, to recovery of clean water, energy, nutrients and possibly other chemicals (metals or pharmaceuticals).
In human waste, most of the nutrients are in the urine, while faeces contains most of the organic matter. The nutrients in your urine could grow all the cereals you eat, while the energy contained in your faeces could power 5% of your household electricity needs. Globally, urine contains almost a quarter of current nitrogen and potassium fertiliser demand, and about 10% of phosphorus used. However, since much Australian agricultural produce is exported, our urine would contribute a much smaller fraction.
The goals of new treatment processes include production of water fit-for-use (for defined purposes), minimisation of energy use (or potentially net energy production) as well as recovery of nutrients.
A number of proposed alternative treatment processes were presented, generally avoiding the energy-intensive aerobic treatment, instead capturing methane from anaerobic digestion, and precipitating magnesium ammonium phosphate (struvite) as a fertiliser product. Electrodialysis can potentially concentrate both nitrogen and potassium as useful nutrients, to support their recovery and avoid the wastefulness of current treatment systems.
Source separation of urine offers further opportunities, keeping the nutrients concentrated, and separating the nutrient and energy recovery processes. This may be processed on-site through electrochemical membrane technologies to recover a solid or concentrated fertiliser product before the water enters the sewage system.
Practical challenges to implementation include providing fertiliser products which compete on price and quality with conventional products, removing heavy metals and pharmaceutical residues, efficient distribution and legislative barriers. However, it is possible to foresee future cities which integrate these technologies, including separated waste streams and decentralised treatment.
About the presenter
Emma Thompson Brewster completed her Bachelor of Environmental Engineering with Honours at UQ in 2012. Upon graduation she worked with the Queensland Department of Energy and Water Supply for six months. She returned to UQ and is now in the third year of her PhD on the topic of recovering nutrients from wastewater using electrodialysis.