Wastewater treatment plants (WWTPs) remain a significant source of nutrients in surface waters. This has also been confirmed through research conducted by Radboud University, which measured greenhouse gas emissions and nutrient concentrations upstream and downstream of six WWTP discharge points [1]. The research showed that greenhouse gas emissions (mainly methane) and concentrations of ammonium, phosphate and total undissolved carbon were higher downstream than upstream of the discharge point. Reducing the concentrations of nutrients can therefore be an important contribution to limit greenhouse gas emissions from water bodies, as well as combat eutrophication.
WWTPs produce large amounts of sludge. Some is used for sludge digestion, but most (97%) is incinerated [2]. Processing and disposal of sludge can account for up to 60% of the operational costs of a wastewater treatment plant [3].
Natural processes and interactions can contribute to sludge treatment: growing macroinvertebrates and aquatic plants on treated and untreated wastewater can reduce sludge, nutrient and greenhouse gas emissions, in addition to producing biomass that can be used in the circular economy. This is Aquafarm [4], [5], [6].
The Aquafarm concept
Aquafarm is a wastewater treatment concept based on naturally occurring processes. The natural processes used in Aquafarm include decomposition of organic matter by macroinvertebrates and nutrient uptake by aquatic plants. The interactions between macroinvertebrates and aquatic plants in a single system can enhance the removal of nutrients. This works best in a cascaded system, with a compartment for sludge reduction using macroinvertebrates, followed by a compartment for nutrient removal using aquatic plants in effluent-fed tanks [3].
The invertebrates grown on sludge from the treatment plant and aquatic plants grown on effluent both purify the water and produce biomass. To optimise nutrient removal, the biomass must be harvested – a process that captures reusable raw components such as phosphorus, which is a limited resource. Wageningen Environmental Research and Radboud University have looked at how these processes work and what they can mean for purification [5], [6]. This research forms the scientific foundation for Aquafarm.
Sludge reduction and nutrient removal
Given the large quantities of sludge being produced, sludge reduction is a much-needed cost-saving solution. There are several macroinvertebrate species that are capable of degrading sludge. They grow by eating organic material. By burrowing in the sludge, they bioturbate (reworking and disruption) the sludge, which improves sludge aeration and reduces the sludge volume. Invertebrates such as tubifex worms, bladder snails and non-biting midge larvae, and combinations thereof, have been used to examine by how much sludge degradation can be accelerated compared to the normal microbial degradation that also takes place in a sedimentation tank. Experiments have shown that these macroinvertebrates can enhance sludge degradation by up to five times (Figure 1). In addition, combinations of different macroinvertebrate species degrade more sludge than expected based on the degradation caused by the individual species [5], [7].
Research [3] has shown that midge larvae reduce sludge by up to 44%, compared to 25% in control experiments (without macroinvertebrates, in which case the sludge is degraded by microbial processes) [3]. Bioturbation by midge larvae also reduces methane emissions from sludge by 92%. Combining midge larvae and aquatic plants ultimately reduces phosphorus concentrations in the effluent by almost twice as much.
Figure 1. Percentage of sludge degradation by three invertebrate species (and all possible combinations): non-biting midge larvae (C), tubifex worms (T) and bladder snails (P), measured at three different WWTPs. Degradation was compared with control sludge containing no invertebrates.
Nutrient removal with floating aquatic plants
The previously mentioned reduction in phosphorus is due to uptake by aquatic plants; however, aquatic plants can also help to remove nitrogen from effluent. Aquafarm research at lab scale has shown that floating aquatic plants, such as common duckweed (Lemna minor) and water fern (Azolla filiculoides), remove nutrients. Submerged plants absorbed nutrients at a relatively slower rate, which gave algae a chance to grow. These algae suppressed the submerged aquatic plants by blocking sunlight from reaching the plants [8]. In situ research spanning a full year has shown that growing floating plants (water fern, common duckweed) effectively remove nutrients from effluent year round. A cascaded set-up of common duckweed followed by water fern proved most effective in terms of nutrient removal.
The growth of floating aquatic plants also leads to carbon capture and provides an oxygen-rich habitat for nitrifying microorganisms. This ultimately leads to lower nitrogen concentrations in the effluent [6]. The research also showed that a cascaded set-up of floating aquatic plants can remove nutrients from effluent year-round, with up to 43% of phosphate removed and 44% of ammonium. These removal efficiencies were determined for a harvest frequency of once every two weeks. For the future use of floating aquatic plants to purify effluent, the harvest frequency will need to be adapted carefully to accommodate growth and thus nutrient removal.
Different plants, different properties
Floating plants were found to remove nutrients from water more effectively than submerged aquatic plants [8]. However, different types of plants can be used for different effluent compositions, as aquatic plants have different properties (Figure 2). Effluent treatment can thus be optimised further based on a treatment task [4].
Figure 2. Eight different floating aquatic plants grown on WWTP effluent. Each plant was tested for its ability to remove phosphate, nitrate and ammonium, growth rate, and limit or absorb the greenhouse gases CO2, N2O and CH4.
Use
The processes described above are effective in laboratory experiments and on a small scale in practice. They provide a scope for removing nutrients from effluent, reducing sludge and combatting greenhouse gas emissions. Moreover, the interactions between macroinvertebrates and aquatic plants have been shown to enhance nutrient treatment efficiency, bringing the Water Framework Directive targets within closer reach. As a low-tech and energy efficient solution to removing nutrients from effluent, Aquafarm has the potential to fulfil this role, thereby reducing the nutrient loads on surface water, preventing the formation of greenhouse gases and even capturing greenhouse gases in the form of aquatic plants.
Aquafarm also has the potential to recover the finite raw material phosphorus through harvesting of the plant material, which can then be used in the circular economy. The harvested material can also conceivably be used in the non-food sector, for example as potting soil, as fibres for insulation material, as enzymes for high-value products such as glue, or as a low-value product in biogas production. Non-biting midge larvae can serve as feed for ornamental fish.
Scaling up Aquafarm
Aquafarm is currently at Technology Readiness Level (TRL) 4, which means that the concept has been shown to work at a lab scale and in a small-scale practical setting. The next step would be to scale up the concept for the further development of additional processes, such as automation and harvesting. In the future, Aquafarm could be a valuable addition to the treatment process when technical resources prove insufficient and natural solutions need to be used instead.
Conclusion
Scientific research has shown that the combination of macroinvertebrates and aquatic plants can play a major role in reducing organic matter and nutrients in primary and secondary wastewater treatment. The Aquafarm concept ensures that these pollutants do not enter the environment. Aquafarm also makes it possible to recover raw materials from wastewater, while the aquatic plants and macroinvertebrates can ensure net carbon capture instead of emission. By using, optimising and combining natural processes with technical treatments, we can adopt a dynamic approach to water quality problems.
Visit www.aquafarm.nl to find out more.
Aquafarm is a natural and low-tech method for the processing of sludge, secondary treatment of effluent and production of biomass, all at the same time, using natural processes. A community of macrofauna breaks down organic matter and aquatic plants absorb nutrients. Wageningen ER and Radboud University have carried out research into whether and how these natural processes can contribute to the future of water treatment. They looked at how macrofauna can reduce sludge and how aquatic plants can purify effluent, as well as whether interactions between these groups influence these processes.