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    The factors affecting bacterial colonisation on microplastics and the impact of tertiary treatment of wastewater on the attached bacteria and microplastics
    (2023-05) Rajcoomar, Saieshna; Bux, Faizal; Kumari, Sheena; Amoah, Isaac Dennis
    Microplastics (MPs) in aquatic environments have become an environmental concern globally. In addition to the direct impact of these plastics on aquatic organisms, their surfaces could serve as a unique habitat for various microbial communities through the formation of biofilms. Various factors could play a role in microbial attachment and biofilm formation in wastewater. This study aimed to assess potential factors that lead to biofilm formation on different types of MPs in wastewater and determine the impact of UV and chlorine treatment on these biofilms. In a laboratory scale experiment, MPs (low density polyethylene (LDPE), high density polyethylene (HDPE), and polypropylene (PP) were exposed to untreated wastewater under various conditions of temperature (20°C, 25°C and 35°C), light and dark conditions, as well as aerobic and anaerobic conditions for a period of five weeks. The formation of biofilms on MPs was quantified using optical density (OD660) measurements. The highest biofilm formation was observed in week 3, with an OD of 1.77. Thereafter, a decline in OD was observed, reaching an OD of 1.1 by week 5. This change in biofilm concentration over the week corresponded to changes in nutrient (nitrite, nitrate and ammonia) concentration in the media. A positive correlation was observed between the changes in biofilm concentration and nitrite (r = 0.824) and ammonia (r = 0.1) levels in the media. Meanwhile, a negative correlation observed for nitrate concentration (r=-0.673). Factors such as dark conditions, 25 C, and aerobic conditions presented the highest median biofilm formation with an OD value of 1.6, 1.7 and 1.6, respectively. It was also observed that polyethylene had higher biofilm concentrations compared to the polypropylene. Furthermore, rough MPs had higher biofilm formation than smooth MPs, with median ODs of 1.7 and 1.6 respectively. The microbial communities in the biofilms and wastewater medium were characterised by 16S rRNA amplicon sequencing. The results revealed that the alpha diversity (richness, evenness, and diversity) was lower in wastewater compared to the biofilms. It was observed that PP supported the most diverse bacterial community ( H’= 2.51138 and Simpson index= 11.096), while HDPE supported the least diverse bacterial community (H’= 0.88779 and Simpson index= 1.5324). Beta diversity using the Jaccard distance index revealed that the most similar communities were observed among biofilms from the three types of MPs while the most dissimilar communities were observed between the biofilm and wastewater medium communities. The most dominant phyla in both the biofilms and wastewater medium during the five weeks were Proteobacteria, Bacteroidetes and Planctomycetes. The bacterial communities, however, varied for each type of plastic and the wastewater medium. It was observed that Methylotenera, Hydrogenophaga, and Rhodanobacter was the most abundant genera in biofilms whereas C39(45.25%) and Luteimonas(18.96%) were the abundant genera in the wastewater medium. Methylotenera mobilis was the most common species among the three types of MPs. In addition, pathogenic species such as Mycobacterium arupense and Methylobacterium adhaesivum were detected in abundance on LDPE and PP. To assess the impact of UV treatment and chlorination on the attached biofilms, the microplastics with attached biofilm were exposed to UV-C and Chlorine (5 mg/L) treatment for 60 minutes. The biofilms were inactivated (100%) after 30 mins of UV treatment, whereas 10 min was sufficient to achieve 100% inactivation of biofilm by chlorine treatment. In conclusion, the research presented in this study has made substantial contributions to our understanding of the role that environmental factors play in the formation of biofilm on MP surfaces.
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    Removal of organic and inorganic nutrients in a constructed rhizofiltration system using macrophytes and microbial biofilms
    (2016) Mthembu, Mathews Simon; Swalaha, Feroz Mahomed; Bux, Faizal
    Many households in developing countries are still without proper sanitation systems. The problems are even more prevalent in rural communities where there are no septic systems in place for the treatment of wastewater. This has resulted in the urgent need for the development and implementation of innovative wastewater treatment systems that are inexpensive, environmental friendly and are able to reduce contaminants to levels that pose no harm to the communities. Constructed rhizofiltration systems have been explored for this purpose. They have been used for many decades in many countries with varying degrees of success at the primary, secondary and tertiary levels of wastewater treatment. Poor optimization of this technology has been due to limited information available about the roles played by the whole system as well as by each component involved in the treatment technology. The current work elucidates the role played by macrophytes and microbial biofilms in the removal of nutrients in the rhizofiltration system. Factors affecting waste removal as well as environmental friendliness of the system were also investigated. The rhizofiltration system was constructed in Durban and was divided into planted (planted with Phragmites australis and Kyllinga nemoralis) and unplanted (reference) section. Dissolved oxygen (DO), pH, water temperature, total dissolved solids (TDS), electrical conductivity (EC) and salinity were monitored. The removal efficiency of nutrients was measured using spectrophotometric methods by measuring the concentration of ammonia, nitrate, nitrite, phosphate and orthophosphate in the wastewater pre- and post-treatment. The total organic carbon, chemical oxygen demand (COD), total Kehldjahl nitrogen, biological oxygen demand (BOD), ammonia, nitrate and the flow rate of wastewater into the system from the settling tank were used for the estimation of carbon dioxide, methane and nitrous oxide emitted from the rhizofilter using the 2009 EPA formulae. Both the planted and reference sections of the system removed nutrients with varying efficiencies. The reduction of nutrients in the rhizofilter was found to be seasonal, with most nutrients removed during the warm seasons. The system also retained more nutrients when wastewater containing low levels of nutrients was used. The unpaired t-test was used to determine the differences between nutrient removals between planted and reference sections. Higher reduction efficiencies of nutrients were obtained in the planted section. Up to 65% nitrite and 99% nitrate were removed while up to 86% total phosphorus was removed in a form of orthophosphate (86%). Removal of total nitrogen was shown to increase under high temperature conditions, while the same conditions decreased the total phosphorus removal. High temperatures also increased the performance of the system. The reduction of nutrients in the system corresponded to reduction of the chemical oxygen demand which also positively correlated to the dissolved oxygen concentration. Considering the discharge limits for all nutrients, the discharges in the effluent of the planted section were within the allowable limits as per South Africa’s Department of Water affairs and Forestry in 2012 but not in 2013. The results obtained in 2013 were due to increased nutrient loading introduced into the system. Diverse microbial communities occurred in the treatment system, with more diversity in the planted section. These organisms were supported by macrophytes in the planted section, and were responsible for nitrogen and phosphorus transformation. This explains why total nitrogen and phosphorus reduction was higher in the planted compared to the reference section. Both the planted and the reference sections of the rhizofiltration system produced the greenhouse gases. When the two sections were compared, the planted section produced more gases. Gases emitted by both sections were lower when compared to emission from sludge treatment reed beds and other conventional systems of wastewater treatments. These findings indicated that constructed rhizofiltration is a cleaner form of waste treatment, producing significantly less greenhouse gases and affecting less of a climate change. Findings of this work have revealed that rhizofiltration technology can be used as a low-cost alternative technology for the treatment of wastewater, using the combination of macrophytes and microbial biofilms. Macrophytes accumulated nitrogen and phosphorus as well as supported diverse microorganisms that metabolized and reduced nutrients in the rhizofiltration unit.
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    Inactivation of Listeria monocytogenes ATCC7 644 Biofilms using Sodium Dodecyl Sulphate, Levulinic Acid and Sodium Hypochlorite solution
    (MN Khan, 2014-06) Singh, Suren; Mnyandu, Elizabeth; Oluwatosin Ademola Ijabadeniyi
    A study was done to assess the effectiveness of 200 ppm sodium hypochlorite (chlorine), 1% sodium dodecyl sulphate (SDS) and 0.5% levulinic acid in reducing L. monocytogenes ATCC7644 biofilms. 0.05% SDS and 0.5% levulinic acid were also used combined (mixture). After treatment with sanitizers, the biofilms were stored at 4°C for up to 72 hours and samples were tested at 0, 24, 48 and 72 hours. The contact times were varied to 1, 3, 5 minutes. Results revealed that biofilms were still viable after treatment with these sanitizers. There was no significance difference between storage times. Varying contact times from 1 to 3 minutes did not show a significance difference however there was a significance difference when the contact time was increased to 5 minutes. Non-adapted biofilms had highest log reductions compared to chlorine adapted and heat adapted biofilms. Treatment with chlorine was least effective in reducing viability of biofilms, followed by levulinic acid then a mixture of levulinic acid and SDS. SDS used alone had highest log reductions. Application of sanitizers at different contact times combined or individually may be successful in reducing biofilms in food manufacturing units. A careful selection of sanitizer for each specific pathogen may be required if sanitizers are to work effectively against biofilms.