Effective removal of ammonia from the drinking water system : a case study at Magalies Water
Mugwili, Muyahavho Enemiah
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Access to clean drinking water and sanitation is recognised as a human right and imperative to holistic functioning of life and interlocking nitty-gritties. In recent years, the quality of surface water in low- and middle-income countries (LMICs) has deteriorated significantly, primarily attributed to upstream activities. Specifically, municipal wastewater treatment processes, agricultural practices, and industrial activities are the main contributors to catchment problems, mainly due to the discharge of effluents rich in nutrients, including ammonia, organic matter, metals, and microbial load. Introducing those contaminants negatively affects the water quality of the catchment, resulting in myriads of challenges such as oxygen depletion, eutrophication, algal bloom, and hyacinth blankets in aqueous environments. In addition to operational challenges associated with nutrient-rich surface water, the efficiency of conventional drinking water treatment systems regarding the removal of ammonia and metals is relatively low, leading to final water with the potential to cause health and aesthetic challenges, impacting the human right to access drinking water. Worryingly, incorporating different technologies into drinking water systems will require structural amendments, which will increase the capital expenditure (CAPEX) and operational expenditure (OPEX), while the environmental impacts limit the capabilities of other technologies. As such, water service providers are searching for cost-effective, safe, and environmentally friendly technologies compatible with existing water treatment systems, hence the purpose of this study. The present study was conceptualised to assess the synergistic potential of integrating struvite synthesis and breakpoint chlorination for the removal of ammonia from drinking water. Specifically, this study is stratified into two step-wise approaches, i.e., batch laboratory (i) and pilot studies (ii) and two treatment steps that comprise struvite precipitation and breakpoint chlorination. In the first study, batch laboratory experiments were conducted to fulfil the goals of this study. Low magnesium and phosphate concentrations in surface water necessitated augmentation of low-cost calcined magnesite as magnesium source, while potassium dihydrogen phosphate provided the phosphate for precipitation with surface water ammonia. Operational parameters comprises the effect of chemical species dosage (magnesium and phosphate), mixing speed, and contact time. State-of-the-art analytical instruments were used to underpin the fate of chemical species. The chemical compositions of the samples were ascertained in an ISO-accredited laboratory following standard methods. Specifically, the laboratory is at Magalies Water Board Laboratory (ISO/IEC 17025:2017 accredited) in Brits, North West, South Africa. Optimum conditions were observed to be 110 mg/L of Mg and P dosage (concentration), 150 rpm mixing speed, 60 minutes contact time and 120 minutes sedimentation. Under these conditions, the pH increased from 6.7 to ≥ 9.6, while the turbidity was reduced from 9.1 to ≤ 1.3 NTU. Approximately 97 of manganese and 99% of iron were removed with ammonia attenuated from 7.60 to ≤ 2.84 mg/L (62.63%). Lastly, in stage 1, the antimicrobial effects on E. coli and total coliform attained ≥ 99% removal efficiency, respectively, while the removal efficiency of TPC was ≥ 87%. The resultant sludge was observed to comprise struvite from XRD and FTIR results, confirming ammonia's fate post-treatment. Product water from this reactor was taken to another stage of water treatment that comprised breakpoint chlorination. In particular, the breakpoint chlorination experiments were conducted using calcium hypochlorite as the chlorine source. Experiments were conducted at an ammonia weight (Cl2:NH3) ratio of 8:1, 200 rpm of mixing speed, and 30 minutes of contact time. Under these conditions, ammonia and total plate count (TPC) removal attained ≥ 99% from aqueous environments. The synergistic and complementary effects of integrating struvite synthesis (precipitation) and breakpoint chlorination completely removed contaminants from drinking water. Albeit there was the formation of trivial disinfection byproducts, mostly chloroform, post the breakpoint chlorination, their levels were below WHO and SANS 241 drinking water standards. Finally, in the second study, the upscaling of the laboratory system into a pilot study was pursued, and cost analysis (OPEX) was also performed. The pilot plant had a capacity of 1000 liters (1 m3) per run (LPR), and optimum laboratory assays were adopted here. Interestingly, results obtained on the pilot scale were similar to those obtained from laboratory assays. Specifically, in Stage 1 of the treatment process, the pH increased from 8.2 to 10.2 while ≥ 99% removal efficacy for manganese and iron was registered. E. coli and coliform reduction was ≥ 99%, while TPC reduction was 91%. Ammonia was reduced from 6.73 mg/L to 2.40 mg/L-N (64.33 %). Furthermore, in Stage 2, i.e., breakpoint chlorination, ammonia was reduced from 2.40 mg/L to ≤ 0.009 mg/L-N after the interaction of water with chlorine, whilst the TPC was reduced from 2340 CFU/1mL to 4 CFU/1mL. The cost evaluation, i.e., OPEX, amounted to R4.85/kl ($0.31/m3) hence comparing favourably to similar studies. Moreover, replacing pure chemicals with chemicals such as waste phosphoric acid can potentially lead to cost savings of R2.58/kl (0.16/m3). As expected, the hybrid process removed contaminants from surface water, yielding product water compliant with WHO and SANS 241 standards and specifications, respectively. Overall, the integration of struvite precipitation and breakpoint chlorination demonstrated synergistic effects toward surface water treatment to the required standard. However, future research should focus on integrating powdered activated carbon into the studied process to remove potential DBPs.
- Engineering