Residual alkalinity left after alkaline regeneration is a critical yet often overlooked factor controlling the long-term performance of iron-oxide adsorbents for phosphorus (P) removal and recovery. In porous adsorbents, incomplete neutralization can maintain a local alkaline environment, promoting calcium (Ca)-based precipitation and performance deterioration. This study evaluated post-regeneration neutralization of the iron oxide adsorbent GEH in Ca- and natural organic matter (NOM)-containing water matrices. Three neutralization conditions were compared: rapid rinsing, thorough washing, and acid-assisted neutralization. Rapid rinsing and thorough washing left residual alkalinity within GEH, driving Ca-based precipitation and progressive loss of adsorption capacity and regeneration efficiency. In contrast, acid-assisted neutralization largely reduced residual alkalinity, suppressed precipitation, and maintained adsorption-dominated P removal. Under acid-assisted neutralization, GEH sustained stable P removal of 4.51–5.83 mg/g with 88%–96% regeneration efficiency over 12 cycles in Ca- and NOM-containing systems. These findings identify post-regeneration neutralization as a preventive control step for residual alkalinity, rather than a delayed corrective treatment for accumulated precipitates, providing a practical strategy to improve the stability of adsorption-based P removal and recovery. ... Read more

Slow sand filters (SSFs) are increasingly recognized for enhancing the biological stability of drinking water. While research has historically focused on the top layer (Schmutzdecke) of SSFs, the contribution of deeper filter depths in removing dissolved organic carbon (DOC) and ammonium (NH4+) has recently been acknowledged. This study investigated the occurrence and potential pathways of DOC release in mature full-scale, and young laboratory SSFs. The top layer (5 cm) reduced the easily biodegradable DOC, mainly low-molecular-weight (LMW) acids and building blocks. The middle layers (20–60 cm) released DOC, particularly LMW acids and neutrals, at depths where nitrification was nearly complete. This release occurred in both mature and young SSFs and may result from bacterial activity under carbon or nitrogen limitation or from the transformation of slowly degradable DOC into labile forms. Whatever the precise mechanism of release, the bottom layers (60–90 cm) subsequently removed this released DOC and reduced PO43– to ultralow levels, highlighting the importance of the deepest layers in maintaining effluent quality. This study provides the first evidence of biodegradable DOC release in SSFs and emphasizes the need to better understand its implications for carbon cycling and removal processes in biological filters. ... Read more

Although many source waterbodies face nitrogen pollution problems, the lack of organic electron donors causes difficulties when aerobic denitrifying bacteria are used to treat micro-polluted water. Different forms of iron with granular activated carbon (AC) as carriers were used to stimulate aboriginal microorganisms for the purification of micro-polluted source water. Compared with the iron-absent AC system, targeted pollutants were significantly removed (75.76% for nitrate nitrogen, 95.90% for total phosphorus, and 80.59% for chemical oxygen demand) in the sponge-iron-modified AC system, which indicated that iron promoted the physical and chemical removal of pollutants. In addition, high-throughput sequencing showed that bacterial distribution and interaction were changed by ion dosage, which was beneficial for pollutant transformation and reduction. Microbial functions, such as pollutant removal and expression of functional enzymes that were responsible for the transformation of nitrate nitrogen to ammonia, were highly efficient in iron-applied systems. This study provides an innovative strategy to strengthen in situ remediation of micro-pollution in waterbodies. ... Read more