In arid and water-stressed regions, groundwater desalination plants are critical for ensuring reliable potable water supplies, making improvements in their operational efficiency and cost effectiveness a priority for utilities. In many such facilities, lime and soda ash softening remain common pretreatment practices, which increase chemical consumption and sludge generation, prompting the need for alternative low-chemical strategies. This study evaluates the technical, operational, and economic implications of transitioning a full-scale brackish groundwater desalination plant, from lime–soda ash softening (old plan) to a low-chemical pretreatment strategy based on antiscalant dosing (new plan) upstream of reverse osmosis (RO). Key parameters, including pH, total hardness, calcium and magnesium hardness, silica, iron, alkalinity, and total dissolved solids (TDS), were measured and compared at multiple locations within the treatment plant under both the old and new plans. Removing lime and soda ash caused higher levels of hardness, alkalinity, and silica in the water before RO treatment, increasing the risk of scaling. Operationally, the feed pressure increased from 11.43 ± 0.16 bar (old plan) to a peak of 25.50 ± 0.10 bar in the new plan, accompanied by a decline in water production. Chemical cleaning effectively restored performance, reducing feed pressure to 13.13 ± 0.05 bar, confirming that fouling and scaling were the primary, reversible causes. Despite these challenges, the plant consistently produced water that complied with Saudi Standards for Unbottled Drinking Water (e.g., pH = 7.18 ± 0.09, TDS = 978.27 ± 9.26 mg/L). Economically, the new strategy reduced operating expenditure by approximately 54% (0.295 → 0.135 $/m³), largely due to substantial reductions in chemical and sludge handling costs, although these savings were partially offset by higher energy consumption and more frequent membrane maintenance. Overall, the findings emphasize the importance of systematic performance evaluation during operational transitions, providing guidance for utilities seeking to optimize pretreatment design while maintaining compliance, long-term membrane protection, and environmental sustainability.

Cationic polymers are critical coagulant aids at drinking water plants, but the same polymers are simultaneously N-nitrosodimethylamine (NDMA) precursors upon chloramine exposure. We hypothesized the low-molecular fraction, which represents a small mass residual in polyDADMAC cationic polymer, reacts with chloramines, may not be well removed during coagulation, and is thus responsible for forming parts-per-trillion (ppt) concentrations of NDMA in finished water. To identify strategies capable of reducing post-coagulation residual polymer associated with NDMA formation, this study fractionated polyDADMAC by molecular weight, characterized reactivity of each fraction, and quantified precursor contributions to NDMA formation potential during bench-scale test and NDMA concentration due to formation during treatment during pilot tests. Diaultrafiltration of the cationic polymer produced a low molecular weight fraction (10 kDa). Native organic matter in bench- and pilot- tests contributed 19–38% of NDMA formed throughout treatment, while polymers were responsible for 62–81% of NDMA formed. The 10× NDMA than non-purified polyDADMAC in jar tests. Purified polyDADMAC, with <10 kDa fraction removed, formed 54% less NDMA during pilot tests (and 63% less during jar tests) than non-purified polyDADMAC. There was no adverse effect on coagulation or subsequent filter performance by using purified polyDADMAC, but significantly (p < 0.05) less NDMA formed in filtered water when using diaultrafiltered cationic polymer. Thus, removing the low molecular weight impurities (<10 kDa) by polymer suppliers would lead to an equally-effective coagulant-aid that substantially lowers NDMA formation during drinking water treatment.

Treated sewage effluent (TSE) is central to Saudi Arabia’s plans to expand irrigation, but assessments of Riyadh’s TSE have relied on period averages. This short-term screening evaluated how reliably three municipal wastewater treatment plants (WWTPs) in Riyadh met the national irrigation limits, using one month of daily operator analyses (April 2026) and two years of daily residual chlorine. Reliability was assessed with the coefficient of reliability, empirical exceedance frequencies and block bootstrapping. Monthly mean five-day biochemical oxygen demand (BOD₅) and total suspended solids (TSS) met the restricted limits at all plants, but no plant met all restricted limits consistently, because the ammonia, turbidity and residual chlorine limits apply to both classes. None met the unrestricted BOD₅ limit: monthly means were 13.7, 23.1 and 18.0 mg/L against 10 mg/L, with ≤1% probability of compliance. One plant did not nitrify (ammonium nitrogen 43.7 ± 2.2 mg/L) and had residual chlorine below 0.2 mg/L on 96% of days since October 2025, whereas the others kept residuals within 0.2–0.5 mg/L on 82–99% of days. Salinity (electrical conductivity 1.7–2.2 dS/m) restricts crops to moderately tolerant species. Nitrification, solids removal and chlorine control are the priority upgrades, and a full year of data is needed to confirm these findings.