Water neutral developments : how to successfully integrate micro-algae systems into wastewater management

Abstract

Treating municipal wastewater is necessary to limit the impact carbonaceous, nitrogenous and phosphorus matter present in spent water may have on receiving aquatic systems. Conventional wastewater treatment systems employing the activated sludge or biological nutrient removal process as the main phase of treatment, demonstrate a high proficiency at removing these contaminants. Despite this, these processes are described as problem shifting, simply causing secondary pollution because of high energy consumed, production of waste sludge and greenhouse gases. To improve the environmental impact of wastewater treatment, particularly in light of stricter effluent discharge standards, treatment processes that have low energy consumption without affecting performance are needed. A potential, more sustainable biological treatment process to remediate the contaminants from wastewater is by using microalgae. Although this concept has been extensively researched, limited commercial development has been achieved. A major hindrance to the implementation of microalgae to treat wastewater is the cultivation process, which is one of the main cost and energy burdens, and as such would not result in the much-desired reduction in overall energy consumption of wastewater treatment. This thesis evaluated the performance of a microalgae treatment process for primary settled municipal wastewater (PSW) in a laboratory setting under static culturing conditions, to examine the feasibility of a low energy treatment process. Initial experiments assessed three freshwater microalga to treat PSW under both optimal (aerated) and static (non-aerated) culture conditions. From these results, Chlorella vulgaris identified itself as the most promising species, exhibiting high inorganic nitrogen and phosphorus removal. The availability of a suitable carbon substrate was determined to be the main limiting-factor affecting the algal treatment performance under static cultivation. To investigate this, initial experiments of PSW enriched with glucose (<300 mg L−1) as an organic carbon source to facilitate the bioremediation by C. vulgaris was performed. Characterisation of the wastewater revealed significant reductions in NH3-N (from 28.9 to 0.1 mg L−1) and PO4-P (from 3.2 to 0.1 mg L−1) in just 2 days. Additionally, the exogenous glucose appeared completely removed from the wastewater after the first day. These achieved levels of treatment in respect of both the NH3-N and PO4-P were much higher than those recorded without C. vulgaris treatment with or without glucose enrichment. The reliability of this process was evaluated across a further three independent batches of PSW with varying compositions and organic carbon sources. The efficiency of the microalgae treatment process at reducing NH3-N and PO4-P was consistent in PSW enriched with organic carbon, resulting in > 90% reduction of the inorganic compounds in each batch. Lastly, to overcome the material cost of applying commercial sources of organic carbon, experiments were conducted to evaluate the use of the carbohydrate rich by-product, pot ale, from the production of malt whiskey as a carbon substrate to promote microalgae growth and remediation in PSW. In batch experiments, repeated three times with wastewater collected and treated separately and sequentially, the efficiency of the microalgae in pot ale enriched PSW demonstrated a high variability at reducing NH3-N and PO4-P, between 99 to 58% and 94 to 58% respectively. When operated under semi-continuous mode the microalgae demonstrated to be reliable in treating pot ale enriched PSW however, the removal efficiency in NH3-N, PO4-P and COD declined slightly in each subsequent cycle following the replenishment of PSW. The results of the pot ale enriched experiments highlight future research needs, such as the optimisation of nutrient ratios in the PSW and control over pH, to ensure a consistent and reliable treatment performance. Overall the application of C. vulgaris to treat enriched PSW, without aeration, offers a key area to develop as an alternative low energy, biological wastewater treatment option