Harnessing immobilised algae for high rate wastewater treatment

Abstract

Algal systems can be used to decrease the concentration of nitrogen (N) and phosphorus (P) in wastewater to low levels, and hence reduce the harm of wastewater discharge and facilitate water reuse. Immobilisation of algae by entrapment in alginate beads can overcome the shortcomings of suspended algal systems of long treatment times, difficulty in harvesting and control of the algal species being cultivated. The aim of this lab-scale study was to investigate and improve nutrient removal from municipal wastewater by alginate-immobilised algae and to explore pathways for its implementation in wastewater treatment plants. Comparison of intensified nutrient removal systems of biofilm, alginate-entrapped, suspended microalgae (Chlorella vulgaris) and macroalgae (Oedogonium cardiacum) demonstrated that the rate of nutrient removal depended more on the respective ability of each system to accumulate algal biomass in the reactor rather than the type of system affecting the rate of nutrient transfer into each unit of biomass. Macroalgae retained with a mesh is a promising option for both nutrient removal and biomass generation that would be simple to operate. The macroalgae produced 102 ± 4 mg/L/d dry weight of biomass and reached levels of 1.3 ± 0.6 mg/L total phosphorus (TP) and 8.5 ± 1.5 mg/L total dissolved nitrogen (TDN) from a feed of 7 and 24 mg/L respectively, and operated for 30 days at a 12 h hydraulic retention time (HRT). Alginate-immobilised algal cells were retained in the reactor more effectively than the other microalgal systems as this system lost only 0.03 times the biomass increase in the reactor compared with 2.6 times for the biofilm and 1.7 times for the suspended systems. However, the alginate-immobilised algal system treated the wastewater for less than 6 days before bead deterioration, meaning frequent bead generation would be needed to initiate new treatment cycles. This would add to the operating costs of the system and therefore was subsequently addressed by alginate product and species selection. To reduce the cost of alginate supply, a non-laboratory product was shown to be a suitable option for immobilisation of C. vulgaris for wastewater treatment and did not negatively impact N and P removal or bead durability. The non-laboratory alginate led to the production of 36 mg/L/d dry weight of algae compared with 47 mg/L/d for the best performing laboratory product, however this was outweighed by raw material cost savings. The sugars of the alginate polymer chain were found to be an important characteristic that governed bead durability, with a high G:M (L-guluronate:D-mannuronate) ratio found to reduce swelling and loss of structural strength when the beads were exposed to wastewater. Investigation of where immobilised C. vulgaris can be implemented within the wastewater treatment train showed that the cells could adjust their internal nutrient content (N of 4.6-7.8%, P of 1.2-3.2%) to a range of nutrient concentrations (TDN of 6.5-54.3 mg/L, TP of 6.2-14.8 mg/L) in several different wastewater sources (two lagoon effluents, secondary effluent samples taken on different dates, and a primary effluent). A strong relationship between the wastewater NH4+ concentration and algal N content (p-value = 0.00004) that was independent of wastewater source demonstrated that the algal cells could adapt well to wastewater with a high NH4+ content and remove more N per cell. Hence, wastewater with a high NH4+ concentration would be a suitable target for algal treatment and would provide extra benefit in N removal. By quantifying abiotic and biotic nutrient removal pathways separately it was determined that wastewater with a high NO3- content led to increased indirect N and P removal and poorer adaptation of algal assimilation to the wastewater N concentration. It was also shown that immobilisation of C. vulgaris did not impact how the algal cells adapted to the wastewater. Comparison of four algal species showed that Scenedesmus abundans could treat the wastewater for longer (42 d) than Chlorella vulgaris (22 d), Coelastrum microporum (8 d) and Selenastrum capricornutum (8 d). The beads of S. abundans were more durable in the wastewater because the algal cells adapted better to growth in the alginate beads and removed nutrients that can damage the Ca-alginate matrix more efficiently. This meant eachbead of S. abundans removed more N (101 ± 8 µg TN/bead) and P (45.9 ± 1.9 µg TP/bead)before needing to be replaced with new beads than did C. vulgaris (41.1 ± 3.8 µg TN/bead, 17.2 ± 0.8 µg TP/bead), Coel. microporum (13.8 ± 3.1 µg TN/bead, 3.7 ± 0.6 µg TP/bead) andSel. capricornutum (14.7 ± 2.3 µg TN/bead, 4.3 ± 0.3 µg TP/bead), enabling more efficient utilisation of alginate.. Differences in the N and P concentration and ratio of the medium used to culture the algae before immobilisation and wastewater inoculation did not impact treatment performance for any of these algal species. Concentrations of ≤ 1 mg TP/L and ≤ 10 mg TN/L were achieved with a fluidised-bed reactor of alginate-immobilised S. abundans treating wastewater with 8.9 mg TP/L and 18.3 mg TN/L at a HRT of 12 h (6 h active bed) for 30 days, and produced 3.3 ± 0.6 x 106 cells/mL/d. Co-digestion of S. abundans biomass without pre-treatment with anaerobic digestor sludge (based on the scenario of separation and reuse of alginate rather than also digesting it) led to biomethane production of 248 ± 10 mL CH4/g VS. Use of alginate-immobilised algae was thus demonstrated to remove the need for alternative P removal systems, to partly offset the amount of N removal needed from other processes, and to produce biomass for energy recovery. This work has contributed to the improvement of alginate-immobilised algae systems for wastewater treatment and demonstrated its technical feasibility for nutrient removal from different wastewaters. The findings can be used to guide how to best implement and integrate alginate-immobilised algae into new and existing wastewater treatment plants and can form the basis for viability assessment of its commercial application