Environmental sustainability opportunity and socio-economic cost analyses of phosphorus recovery from sewage sludge

Tailoring pore size to enhance dissolution of layered double oxides for efficient nitrogen and phosphorus recovery via crystallization of struvite from wastewater

Nitrogen and phosphorus recovery from wastewater to produce struvite fertilizer can simultaneously alleviate water eutrophication and phosphorus scarcity. Coupled magnesium mineral dissolution and reprecipitation has been emphasized for struvite formation by avoiding excessive Mg salts addition and pH adjustment. However, the recovery efficiency is limited by the low solubility of magnesium mineral. Herein, we capitalized on the concept of "pore-size controlled solubility (PCS)" to enhance the dissolution of MgAl-layered double oxide (MgAl-LDO), where dissolution rate is inversely related to pore size. By reducing the pore size of LDOX from 9.2 to 5.5 nm, the magnesium release from LDOX dissolution increased from 64.0 to 99.0 mg/L, accompanied by OH- release to maintain an alkaline condition. This resulted in effective recovery of NH4-N (80.54%) and PO4-P (80.57%) as high-purity struvite. Scanning electron microscope revealed that LDO acted as seeded surfaces that significantly reduced struvite nucleation barrier by controlling local supersaturation and interfacial energy. Moreover, the recovery is stable across wide pH ranges (5.0-10.0), attributable to the buffering capacity of Al in LDOX. Finally, the feasibility of NH4-N and PO4-P recovery was successfully applied to real aquaculture wastewater. This study offers an effective strategy to enhance mineral dissolution-precipitation for resource recovery.

An analytic hierarchy process combined with artificial neural network model to evaluate sustainable sludge treatment scenarios

Sludge management in China faces critical environmental, economic, and technical challenges, necessitating urgent optimal management strategy selection. Given the limited number of comprehensive studies on sludge management, quantitative decision-making tools are urgently required. To address this gap, this study developed an integrated Analytic Hierarchy Process (AHP)-artificial neural network (ANN) model to evaluate sludge treatment scenarios. Four representative scenarios were evaluated based on carbon emissions, environmental impact, and economic costs. A hierarchical evaluation model based on the AHP was established for sludge treatment processes. Weight indicators were derived through expert questionnaire surveys and combined with empirical data to determine the comprehensive weights. The bootstrap method was applied to expand the sample size and ensure robust training of the ANN model. The ANN framework establishes mapping relationships between evaluation indicators and expected values. The AHP-ANN evaluation model demonstrated high predictive accuracy, achieving a maximum mean squared error (MSE) of 0.00052 in the test dataset. This model enabled the rapid assessment of parameter adjustments on evaluation outcomes and provided a quantitative basis for engineering optimization. Among the evaluated scenarios, the anaerobic digestion scenario (S1) demonstrated the best overall performance, characterized by low environmental impact and operational costs. Conversely, the incineration scenario (S3) exhibited the poorest overall performance, with high resource consumption, resulting in significant environmental impact and elevated operational costs.

Spatial and temporal dynamics of sewage sludge phosphorus recovery potential in the cities of Yangtze River Zone in China: Implications for regional recycling policies

Sewage sludge phosphorus (P) recovery presents opportunities to sustainably recycle P from cities to agriculture and alleviate global P scarcity. However, limited research explores sustainable recovery targets considering spatial-temporal variations in sludge generation and implications based on city-level local P demand. This study analyzed sludge production form 2009-2021 across 130 cities in China's Yangtze River Zone, which increased by almost 35 % from 2009 to 2021. Per capita gross domestic product (GDP), influent chemical oxygen demand (COD), and per capita drainage infrastructure were identified as the main significant influencing factors. City-level analysis revealed pronounced spatial-temporal disparities, with yearly sludge generation spanning five orders of magnitude (62-5.4 × 105 t/a). An indicator, "Potential of P recovery to local P demand", was defined, indicating the average city-level P recycle contribution increased from 5.3 % to 18.9 % from 2009-2021. A novel frame paradigm classified cities into six types based on the local P supply-demand characteristics, prioritizing sludge P recovery and implementing strategic management. City-specific dynamics and possibilities of broader "city clusters" to match supply and demand should be considered for policy implement. Recovering P from livestock manure and kitchen waste alongside sludge can further strengthen urban P cycles. This study provides novel city-scale analysis and strategic considerations for regional sludge P recycling policies in China and beyond.

A pilot-scale evaluation of residual sludge quality in a worm-sludge treatment reed bed in the Mediterranean region

A pilot-scale study on sludge treatment reed beds investigated the combined effects of earthworms and Arundo donax on sewage sludge dewatering and residual sludge quality. Four units were tested: one planted with earthworms, one planted without earthworms, one unplanted with earthworms, and one control, each unit replicated. Over a year, 24 cycles of sludge (dry and volatile solid contents of 24.71 g.L-1, and 19.14 g.L-1) were fed onto the units at a sludge loading rate: 43.59 kg.DS.m-2.year-1. Afterward, the units experienced 132 days of resting period, increasing dry solids from 21 to 70 % and decreasing volatile solids from 81 to 69 % on average (40 % sludge volume reduction). The bottom layers of the planted unit with earthworms showed a 30 % reduction in volatile solids, indicating improved sludge stabilization. Macronutrient abundance in the residual sludge followed the sequence N > Ca > P > K > S > Mg. The planted unit with earthworms reduced micronutrient concentrations by 22 % compared to the control unit (Fe > Na > Mn > B > Mo). Earthworms also played a key role in reducing heavy metal concentrations by 11 % compared to the planted unit without earthworms (Zn > Cr > Pb > Ni > Cd). Heavy metal levels in the residual sludge met EU and Portugal standards, with a 99.9 % reduction in Escherichia coli and fecal coliforms. Cost estimation showed centrifugation and W-STRB scenarios cost 167 and 183 €.PE-1 for a ten-year operation, with O&M costs of 7 and 3 €.PE-1.year-1, respectively.

Potassium and ammonium recovery in treated urine by zeolite based mixed matrix membranes

Nitrogen, phosphorus and potassium are essential for crop growth, which are abundant in urine. Although numerous studies have developed techniques to recover ammonium and phosphorus from urine, limited research made efforts on the recovery of potassium, which is a non-renewable resource with uneven global distribution. In this study, we explored the possibility of zeolite based mixed matrix membranes (MMMs) to selectively recover ammonium and potassium from urine, with minimal detention of sodium. The findings demonstrated that upon the pre-treatment of zeolites with sodium chloride solution, a 70 wt% zeolite loaded MMM could achieve 69.3 % recovery of potassium and almost full recovery of ammonium. By varying the desorption temperatures and MMMs production process, it was discovered that stepwise backwash at low temperature (276 K) greatly lowered sodium recovery whilst simultaneously enhancing the recovery of potassium and ammonium. This study demonstrates the potential of recovering potassium and ammonium from urine using zeolite-loaded MMMs, coupled with achieving low-sodium recovery.

Opportunity Analysis of Phosphorus Recovery from Municipal Wastewater for Cropland Based on the Simulated Vehicle Transport Distance in the Yangtze River Delta, China

With the rapid depletion of phosphate rocks and increasing agricultural demand, establishing a phosphorus (P) flow "loop" rather than a one-way trajectory between cropland and urban areas was imperative. Recovering P from municipal wastewater stood as a viable strategy to mitigate reliance on traditional P-containing chemical fertilizer. This study analyzed the intricate relationships between the potentials of P recovery from municipal wastewater and the P demand of croplands in the populated Yangtze River Delta (YRD), China. An indicator of the P vehicle transport distance was constructed and calculated to estimate the potential to recover and reuse P in agriculture, applying the simulated annealing (SA) algorithm and road networks obtained from OpenStreetMap (OSM). The results indicated that, on a regional scale, recovered P from municipal wastewater could fulfill 14.0% of the cropland P demands in the YRD, with a median P vehicle transport distance of 3.1 km/Mg of P. Notably, the P vehicle transport distance varied largely depending upon the cropland distributions, road density, and P recovery potential from municipal wastewater. The novel methodology developed here determined the optimal transportation routes for P recovery from wastewater treatment plants (WWTPs) to cropland, which played a crucial role in refining the wastewater management strategies aligned with the United Nations Sustainable Development Goals.

Biochar derived from alkali-treated sludge residue regulates anaerobic digestion: Enhancement performance and potential mechanisms

Biochar produced from bio-wastes has been widely used to promote the performance of anaerobic digestion. Waste activated sludge (WAS) is considered as a kind of popular precursor for biochar preparation, but the abundant resources in WAS were neglected previously. In this study, the roles of biochar prepared from raw, pretreated, and fermented sludge on anaerobic digestion were investigated. That is, parts of carbon sources and nutrients like polysaccharides, proteins, and phosphorus were firstly recovered after sludge pretreatment or fermentation, and then the sludge residuals were used as raw material to prepare biochar. The methane yield improved by 22.1% with adding the biochar (AK-BC) prepared by sludge residual obtained from alkaline pretreatment. Mechanism study suggested that the characteristics of AK-BC like specific surface area and defect levels were updated. Then, the conversion performance of intermediate metabolites and electro-activities of extracellular polymeric substances were up-regulated. As a result, the activity of electron transfer was increased with the presence of AK-BC, with increase ratio of 21.4%. In addition, the electroactive microorganisms like Anaerolineaceae and Methanosaeta were enriched with the presence of AK-BC, and the potential direct interspecies electron transfer was possibly established. Moreover, both aceticlastic and CO2-reducing methanogenesis pathways were improved by up-regulating related enzymes. Therefore, the proposed strategy can not only obtain preferred biochar but also recover abundant resources like carbon source, nutrients, and bioenergy.