Municipal Wastewater Treatment Using Microalgae-Bacteria (MaB) Flocs in the Reduced Hydraulic Retention Time (HRT)

Abstract

Increase of the population of the Earth and the drastic rate of urbanization is associated with production of enormous amount of municipal wastewater. The sixth goal of the UN Sustainable Development Goals (SDG) is to “ensure availability and sustainable management of water and sanitation for all”. Sustainable sanitation is one of the global challenges for our societies and environment which is out to be achieved by 2030 (Qadir et al., 2020). Wastewater treatment plants (WWTP) have a key role in decontamination of the municipal wastewaters in large scales and fulfil the sixth SDG. The conventional treatment systems are dependent on the activated sludge process. This process is heavily aerobic, thus requires intense mechanical aeration. Therefore, to operate the conventional treatment systems, high and constant supply of energy is vital. There are also some other concerns about the environmental impacts of the conventional treatment systems. The microalgal photosynthetic oxygenation can be introduced as a sustainable substitute for the mechanical aeration of the activated sludge. Not only microalgae can remove the nutrients effectively, but the algae and bacteria of the activated sludge can form aggregates (flocs) to recycle their produced and required oxygen (in exchange for CO2). Since the application of algae biotechnology demands a relatively large surface area, optimizing the wastewater treatment performance is crucial to minimize the land use. This experimental research is an attempt to address the challenges in reducing the hydraulic retention time (HRT) of the MaB treatment in a single stage. Offering an empirical mechanism to reduce the HRT to 2 days and shorter is one of the main objectives of this study. This study led to publication of 2 articles in the scientific journals. The first article (addressed as Article 1 in this dissertation) is titled ‘Influence of hydraulic retention time on municipal wastewater treatment using microalgae-bacteria flocs in sequencing batch reactors’. Article 1 is published in the journal of Bioresource Technology Reports (issue 17). In this article, a MaB culture consisting of a consortium of indigenous algae and activated sludge was used to treat real municipal wastewater. It was observed that after exhaustion of the organic carbon (OC) by the heterotrophic bacteria (mainly during the first illumination period), the surplus oxygen was employed to nitrify the leftover ammonium on the second day. The highest biological nitrogen and phosphorus removal took place on the first day of the treatment before the nitrification accelerates. Removal of the produced nitrates (NOx) to the EU limits was the main challenge to reach the European standards within 2 days of HRT. Extension of the treatment to 3 days did not improve the nutrients uptake as the elevation of the pH (due to photosynthesis and shortage of alkalinity) triggered a series of inhibitions to the MaB microorganisms. It is assumed that the high pH and subsequently raise of the free ammonia (FA) concentration could cause the nitrate accumulation, inhibition of nitrification, and a lasting suppression of the heterotrophic and photoautotrophic activities. Disintegration of MaB flocs and culture collapse was observed in the sequencing batch reactors (SBRs) with 3 days of HRT. The second article (Article 2) with title ‘Influence of supplementary carbon on reducing the hydraulic retention time in microalgae-bacteria (MaB) treatment of municipal wastewater’ is published in the Journal of Water Process Engineering (issue 51). This article presents the result of the extensive experiments which were designed to prevent the exceedingly alkaline culture conditions, and furthermore, impede the formation of nitrates. To negate raise of the pH (<8.0) and maintain the alkalinity, regulated addition of exogenous (CO2) was practiced. In absence of high pH and FA levels, the activity of the nitrifying bacteria increased, and more nitrates could be produced on the second day. Due to availability of ammonium, algae could not remove the nitrate within 2 days of HRT. Two sequencing batches of 3 days HRT could however allow the EU standards to be satisfied by giving the algae enough time to assimilate the remaining nitrate and phosphorus on the third day. Article 2 experiments continued with examination of the effect of supplementation of the municipal wastewater with OC (glucose) to enhance the MaB treatment. The stoichiometrically balanced COD:N ratio (w:w) of roughly 16 is calculated to boost the internal cycle of C+O2 ⇔CO2. The results showed that the exogenous supplementation with OC is a significantly reliable approach to improve the assimilation of both N and P within about 24 hours while no external auxiliary aeration. The settling quality and quantity of the harvested MaB biomass was also higher in this method. Because large scale utilization of glucose to supplement the municipal wastewater treatment is not economically and environmentally sustainable, the potential waste-sourced alternatives were tested (unpublished results). Glycerol, lawn silage (LS) extract and the effluent from the lautering process of a commercial beer brewer (BBL) were the wastewaters with high COD:N ratios. From among these substrates, the latter (beer brewing wastewater) could show a comparable performance with the glucose and can be introduced as a sustainable alternative for exogenous supplementation of the organic carbon to reduce the HRT of the MaB treatment of municipal wastewater to less than 28 hours.