Harnessing waterfleas for water reclamation: A nature-based tertiary wastewater treatment technology

Urbanisation, population growth, and climate change have put unprecedented pressure on water resources, leading to a global water crisis and the need for water reuse. However, water reuse is unsafe unless persistent chemical pollutants are removed from reclaimed water.. State-of-the-art technologies for the reduction of persistent chemical pollutants in wastewater typically involves high operational and energy costs and potentially generates toxic by-products (e.g., bromate from ozonation). Nature-base solutions are preferred to these technologies for their lower environmental impact. However, so far, bio-based tertiary wastewater treatments have been inefficient for industrial-scale applications. Moreover, they often demand significant financial investment and large infrastructure, undermining sustainability objectives. Here, we present a scalable, low-cost, low-carbon, and retrofittable nature-inspired solution that could be retrofitted into current wastewater treatment systems to remove persistent chemical pollutants. The technology uses the water flea Daphnia to non-selectively uptake and retain persistent chemical pollutants (pharmaceutical, pesticides and industrial chemicals). We showed Daphnia’s removal efficiency at laboratory scale ranging between 50% for PFOS and 90% for diclofenac. We validate the removal efficiency of diclofenac at prototype scale showing sustained performance over four weeks in outdoor seminatural conditions. A techno-commercial analysis on the Daphnia-based technology suggests several technical, commercial and sustainability advantages over established and emerging treatments at comparable removal efficiency, benchmarked on available data on individual chemicals. Further testing of the technology is underway in open flow environments holding real wastewater. The technology has the potential to improve the quality of wastewater effluent  meeting requirements to produce water appropriate for reuse in irrigation, industrial application, and household use. By preventing persistent chemicals from entering waterways, this technology has the potential to maximise the shift to clean growth, enabling water reuse, reducing resource depletion and preventing environmental pollution.

Methods
A total of 216 exposures were completed to quantify the removal efficiency of four classes of chemicals-an industrial chemical (PFOS: 70ng/L; CAS:2795-39-3), a biocide (atrazine:0.2 mg/L; CAS:1912-24-9), a pharmaceutical (diclofenac: 2 mg/L; (CAS:15307-79-6) and a heavy metal (arsenic 1 mg/L; CAS: 7784-46-5), by four genotypes of Daphnia manga: DM1900; DM1960; DM1980; and DM2015. Following an acclimation phase, the removal efficiency of the four genotypes was quantified following exposure to individual chemicals and chemical mixtures from wastewater. the removal efficiency was quantified for PFOS, diclofenac and atrazine over three days with ultraperformance liquid chromatography (UPLC) coupled with high-resolution mass spectrometry. Arsenic samples were quantified using a Nexion 300X inductively coupled plasma mass spectrometer (ICP-MS) (PerkinElmer, Seer Green, U.K) fitted with a cyclonic spray chamber. The removal efficiency of the chemicals was calculated as [starting concentration - final concentration / starting concentration] x 100. Removal efficiency variation by genotype and day was assessed through an ANOVA analysis using the Satterthwaite's method (lmerTest package in R).

We assessed removal efficiency of other inorganics from secondary treated wastewater by Daphnia in laboratory conditions over a period of 3 weeks. Using the HACH colorimeter, we tested removal efficiency of ammonia, total phosphorus and nitrogen, chemical oxygen demand (COD), suspended solids and pH. This quantitation was done on wastewater sourced from the Finham treatment plant from both aeration tanks and the secondary clarifiers. After collection, the wastewater was equally split in triplicate 20L aquaria and inoculated with Daphnia at a density of 100 individuals/L. The inorganics were quantitated twice a week over 3 weeks.

Having identified the strains with the highest removal efficiency in the laboratory exposures, we tested removal efficiency of a population of these strains in a prototype of the technology in outdoor conditions for four weeks to test whether the removal efficiency at scale was comparable to the one measured in the laboratory experiments and to measure performance over time. The prototype was filled with borehole water spiked with diclofenac, a common anti-inflammatory present in wastewater. Removal efficiency was tested daily over a period of 4 weeks using the same approach described for laboratory-based experiments.