The project involved the design, installation and one-year operation of a High-Rate Algal Pond (HRAP) trial at Beckington Sewage Treatment Works, as part of UKWIR’s Innovative Phosphorus Trials. HRAPs are the cheapest and most robust algal growth system however they have not been tested to date in realistic conditions in the UK. The main purpose of this trial was to investigate whether algal HRAPs could be used in UK conditions and at scale for phosphorus removal. The HRAP trial was located at Beckington STW, a trickling filter system with no other phosphorus removal and was fed final effluent from this plant.

Science

The HRAPs consisted of a header tank, filled with final effluent, two parallel HRAPs and two parallel conical settling tanks. Samples were taken from the header tank (HT - “influent”) and the two settling tanks (ST1&2 – “polished effluents”) after ~ 8 (± 1) hours of settling. HRAPs were seeded with a novel, autoflocculating strain of algae termed Av12. The main innovative feature of this trial was to test the performance of autoflocculating algal strain Av12 in realistic conditions. The benefit of using this strain is that it can settle at rates similar to activated sludge and so conventional separation techniques can be used without the need for flocculant or other advanced separation technologies. The autoflocculating behaviour was lost once during the trial but was recovered by increasing the % daily effluent exchange. Av12 was not able to remain dominant throughout the year. There were four instances were a culture crash of Av12 resulted in a mixed community becoming dominant. In total, Av12 was dominant through 49.5 % of the trial’s duration. At commissioning, it was found that mixing was inadequate to keep Av12 in suspension. This led to 25 % of the trial period (mid-Sept to mid-Dec 2016) being run with a mixed algal community in suboptimal conditions until the mixing could be improved. Effluent TP was found to be as low as 0.14 mg/L but varied considerably with the average being 1.98 mg/L and the highest value being 4.5 mg/L. When Av12 was dominant the above values were 0.14, 1.2 and 3.1 mg/L respectively. Minimum SRP was 0.01 mg/L but removal varied with the average being 1.34 mg/L and highest value being 4.0 mg/L. When Av12 was dominant the same values were 0.01, 0.57 and 1.77 mg/L respectively.

The performance of settling of the HRAP community varied considerably with minimum, average and maximum TSS values 4, 39.5 and 115 mg/L respectively. When Av12 or other autoflocculating algal strains were dominant (two such strains were discovered during the trial), the minimum, average and maximum values for TSS were 4, 15.5 and 35 mg/L respectively. While a more conventional settling tank with a weir system would likely improve settling performance, more work is needed to improve the robustness of autoflocculation in strain Av12 as well as the stability of the algal/microbial community.

Impact

Overall, the HRAP trial showed promise as a system that, when performing in a stable mode, can reduce TP and SRP levels to < 1 mg/L and < 0.5 mg/L respectively while removing phosphorus in a form where it can be reused (fertiliser/feed) and having minimal impact on the environment. Furthermore, the productivities seen in this trial exceeded those of the most productive land-based crops in terms of protein and calorie production, even in suboptimal climate. However, issues with settling performance and community stability mean that further optimisation would be necessary before the full benefits of such a system can be realised. It is also important to note that such a system is likely to need a significant level of automation and operational supervision (akin to SBR facilities) to ensure optimisation of robustness and performance.