Phosphorus recovery potential from sewage sludge by struvite precipitation: remodelling policy framework in Rajasthan, India.

The manufacturing of fossil-based fertilizers by extraction of rock phosphate has contributed to carbon emissions and depleted the non-renewable phosphorus reserves. Sewage sludge, which is a waste product from Sewage Treatment Plants (STPs), is rich in phosphorus. The existing techniques for sludge management contribute to carbon emissions and ecological footprint. Struvite (raw fertilizer) and biochar recovery from sludge has emerged as viable methods to reduce carbon emission and ensure economic sustainability of STPs. In this work, the potential for phosphorus recovery and revenue generation is discussed for Rajasthan state in India. The fate of phosphorus and heavy metals in STPs is evaluated which indicates that about 70% of the phosphorus and trace amounts of metals end up in sewage sludge. Further, the power consumption is high in STPs due to industrial wastewater ingress. There is a need to bridge the gap between sewage treatment and generation in Rajasthan, improve STP performance before resource recovery inclusion at policy-level and scale-up. Mixing struvite with biochar can lead to safe application of struvite as raw fertilizer as heavy metals are sequestered by biochar. A business framework is developed to serve as a blueprint and potential model for linking technical and market viability.

High rates of nitrogen removal in aerated VFCWs treating sewage through C-N-S cycle.

The efficiency of deep aerated vertical flow constructed wetlands (DA-VFCWs) being operated in Hyderabad, India, was evaluated herein using physicochemical analysis and 16S rRNA amplicon sequencing. The results showed 2-4-fold higher removal rate coefficients for Biochemical oxygen demand (1.32---3.53 m/d) and nitrogen (0.88--1.36 m/d) in DA-VFCWs than those of passive VFCWs. Elevated sulfate concentration in the DA-VFCWs effluent (84-113 mg/L) indicated possibility of sulfur-driven autotrophic denitrification (SDAD) as a major pathway operating in these wetlands besides the classical nitrogen removal pathways. The presence of nitrifiers (3.09-10.02 %), heterotrophic and aerobic denitrifiers (0.79-0.83 %), anammox bacteria (1.31-2.22 %) and SDAD bacteria (0.08-0.73 %) in the biofilm samples collected from the DA-VFCWs exemplify an interplay of Carbon-Nitrogen-Sulfur cycles in these systems. If proven, the presence of an operational SDAD pathway in DA-VFCWs can help reduce surface area requirement in VFCWs substantially besides alleviating biological clogging of the wetland substrate.

Optimization of depth of filler media in horizontal flow constructed wetlands for maximizing removal rate coefficients of targeted pollutant(s).

Varying the depth of HFCW media causes differences in the redox status within the system, and hence the community structure and diversity of bacteria, affecting removal rates of different pollutants. The key functional microorganisms of CWs that remove contaminants belong to the phyla Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes. Secondary data of 111 HFCWs (1232 datasets) were analyzed to deduce the relationship between volumetric removal rate coefficients (KBOD, KTN, KTKN, and KTP) and depth. Equations of depth were derived in terms of rate coefficients using machine learning approach (MLR and SVR) (R2 = 0.85, 0.87 respectively). These equations were then used to find the optimum depth for pollutant(s) removal using Grey wolf optimization (GWO). The computed optimum depths were 1.48, 1.71, 1.91, 2.09, and 2.14 m for the removal of BOD, TKN, TN, TP, and combined nutrients, respectively, which were validated through primary data. This study would be helpful for optimal design of HFCWs.

Assessment of removal rate coefficient in vertical flow constructed wetland employing machine learning for low organic loaded systems.

Secondary datasets of 42 low organic loading Vertical flow constructed wetlands (LOLVFCWs) were assessed to optimize their area requirements for N and P (nutrients) removal. Significant variations in removal rate coefficients (k20) (0.002-0.464 md-1) indicated scope for optimization. Data classification based on nitrogen loading rate, temperature and depth could reduce the relative standard deviations of the k20 values only in some cases. As an alternative method of deriving k20 values, the effluent concentrations of the targeted pollutants were predicted using two machine learning approaches, MLR and SVR. The latter was found to perform better (R2 = 0.87-0.9; RMSE = 0.08-3.64) as validated using primary data of a lab-scale VFCW. The generated model equations for predicting effluent parameters and computing corresponding k20 values can assist in a customized design for nutrient removal employing minimal surface area for such systems for attaining the desired standards.

Management of reject water in decentralized community RO plants by devising an integrated treatment scheme of NF and RO by pilot-scale analysis.

Community RO plants have been installed in the semi-arid state of Rajasthan in India to provide potable water to the scattered rural settlements by treatment of brackish groundwater. Presently, these are using standalone RO systems which are operating at low recovery along with the problem of early membrane scaling. To ensure sustainability and maximize the recovery of fresh water, hybrid configurations of membrane processes must be evaluated. In this work, it is aimed to design a conclusive hybrid scheme of NF and RO to deliver maximum freshwater recovery. Firstly, the individual performance of NF and RO in a two-pass NF-RO configuration is evaluated i.e., the removal of ions with respect to feed concentration, ionic radius and hydration radius. The removal efficiency was 85% for sulphate, 54% for calcium and 56% for magnesium by NF. The scaling potential of the water greatly reduced as indicated by the LSI and RSI values by NF pre-treatment. The characterization of RO and NF by FESEM-EDS and FTIR Spectroscopy showed numerous peaks in NF as compared to RO corresponding to inorganic scaling. The specific energy consumption for NF, RO and two-pass NF-RO was 0.13-0.27 kWh/m3, 0.04-0.08 kWh/m3 and 0.17-0.35 kWh/m3, respectively. Based on the performance of standalone NF, RO and two pass NF-RO, mathematical simulations were performed to derive best configurations for NF-RO integration. The resulting configuration, a two stage RO-NF with NF permeate blending to the raw water, resulted in a recovery of 70-80% which was ∼50% higher than the two-pass NF-RO scheme.

Strategies for enhancing phosphorous removal in Vertical Flow Constructed Wetlands.

Constructed wetlands (CWs) are among the fastest emerging treatment methods for wastewater treatment. Unlike their organics and nitrogen removal capacities, the potential of CWs as a sink for phosphorous is still debatable. In this study, the secondary data from several CWs treating domestic sewage were compiled and assessed. Curves were plotted between orthophosphate-phosphorous (PO43--P) loading and the corresponding removal rates. Other factors affecting PO43--P removal like depth of the CW, surface area, organic loading rate etc. Were also analyzed. Removal rates of PO43--P were conforming to a linear positive relation with the loading rates. Pea gravel as a CW medium performed consistently well (60-80% removal) for a wide range of influent PO43--P loading (0.5-1.5 g/m3-d). The increased depth of the wetland appears to favor phosphate removal. PO43--P removal was found to be correlated with outlet dissolved oxygen, total Kjeldahl nitrogen removal and effluent nitrate. The study suggests that proper design, optimal organic loading and suitable pre-treatment may increase the applicability of CWs for phosphate removal from domestic wastewater. Larger area requirements can also be avoided by increasing their depth while keeping the volume of the filter media the same.

Designing the vertical flow constructed wetland based on targeted limiting pollutant.

The requirement of large land area limits the adoption of constructed wetlands (CWs) in urban settings with limited land availability. The area calculations for CW design are commonly carried out following Kikuth approach where the removal rate constant (K) is derived from literature. Investigation of secondary data of 82 vertical flow CWs, performed in this study, yielded wide variations (0.0003 - 0.822 md-1) in the calculated K values for different pollutants under different environmental and operational conditions indicating that it is important to incorporate the desired levels of pollutant removal to arrive at customized design of CWs. The results indicated that the relative standard deviation of K values could be narrowed by classifying the datasets based on design parameters like depth, hydraulic loading rates and substrate loading rates. These calculations can help arrive at more scientific design of CW to achieve the prevailing standards for the discharge or reuse of sewage.

Novel microbial nitrogen transformation processes in constructed wetlands treating municipal sewage: a mini-review.

Traditionally nitrogen transformation in constructed wetlands (CWs) has been attributed to the activities of aerobic autotrophic nitrifiers followed by anoxic heterotrophic denitrifiers. However, the nitrogen balances in such systems are far from being explained as a large fraction of the losses remain unaccounted for. The classical nitrification-denitrification theory has been successfully employed in certain unit processes by culturing fast-growing bacteria, but the CWs offer an ideal environment for slow-growing bacteria that may be beneficially exploited to achieve enhanced nitrogen removal by manipulating the environmental conditions in their favor. In the last three decades, many novel microorganisms have been isolated from CWs that have led to the discovery of some other routes that have made researchers believe could play a significant role in nitrogen transformation processes. The increased understanding of novel discerned pathways like anaerobic ammonium oxidation (ANAMMOX), heterotrophic nitrification and aerobic denitrification, which are mediated by specialized bacteria has indicated that these microorganisms could be enriched by applying selection pressures within CWs for achieving high rates of nitrogen removal. Understanding these novel nitrogen transformation processes along with the associated microbial population can provide new dimensions to the design of CWs for enhanced nitrogen removal.

Performance assessment of pulsating floc blanket clarifiers and conventional clariflocculators in pilot-scale models.

Continuous upflow pilot plants based on conventional clariflocculation (CC) and pulsating floc blanket clarification (PFBC) technologies were designed and fabricated for a capacity to treat about 8,000 L/day, to understand the fundamental differences in their functioning and assess their relative performance, especially under low turbidity conditions. Influent turbidity varying from 2 to 10 NTU was treated using coagulant alum, and efficiency of CC and PFBC in terms of average turbidity removal was found to be 23% and 48%, respectively. On observing this vast difference, it was postulated that total residual aluminum should also be lower in water treated from PFBC. Experiments and MLR analysis confirmed the hypothesis, with residual aluminum ranging from 0.055 to 0.040 mg/L and 0.036 mg/L to below detectable levels for CC and PFBC, respectively. These findings are of high significance, since minimization of residual aluminum in drinking water is a priority of WHO due to its reported neurotoxicity and can be complied with simple replacement of CC with PFBC. PRACTITIONER POINTS: Pulsating floc blanket clarifier (PFBC) performed better than conventional clariflocculator (CC) in terms of turbidity removal. Pulsating floc blanket allowed more effective utilization of coagulant alum, resulting in significantly lower residual aluminum in clarified water. Turbidity levels of influent and effluent are related to residual aluminum in treated water. PFBCs are more compact and modular, and can facilitate a good alternative to CCs.

Experimental investigation and modelling the effect of humic acid on coagulation efficiency for sludge blanket clarifier.

The physicochemical process of coagulation has largely been used for turbidity removal for water treatment. However, lately, the intrusion of NOM (Natural organic matter) in the surface water sources due to climate change has impeded the dosing approaches and has presented a requirement to evaluate the effect of NOM on turbidity removal efficiency and the performance of coagulation reactors in general. In this work, a previously developed performance model for hydraulic flocculators was modified and tested for a sludge blanket clarifier (SBC) which is a type of hydraulic flocculator. The experimental runs were conducted by preparing synthetic sample waters by using humic acid (for NOM) and kaolin clay (for turbidity). PACl (Poly aluminium chloride) was used as a coagulant. The expression of attachment efficiency has been modified to include the interactions of humic acid (HA) and kaolin, which were not previously accounted for in the model. The coverage functions were used to calculate the attachment efficiency of HA-PACl and PACl-Clay. The standalone coverage function ГPACl-HA efficiently predicted the doses where the removal efficiency was maximum. However, the coverage function ГClay-PACl was impacted by the hydrodynamic conditions in SBC and over-speculated the Clay-PACl interactions. The RMSE value was low for the modified equation indicating that in SBC the interactions between the organic and inorganic impurities are significant. The HA-Kaolin interactions were found to be significant in the modified model in case of a low HA range of 4 and 8 mg/L of HA.

Nitrogen transformation processes and mass balance in deep constructed wetlands treating sewage, exploring the anammox contribution.

This work was aimed to assess the contribution of classical nitrogen removal pathways in two deep constructed wetlands CW1 and CW2 located at Jaipur, India. Nitrogen mass balance revealed that 44.87% and 43.77% losses of T-N in CW1 and CW2 were unaccounted for. To elucidate these significant losses, the study was extended to assess the occurrence and contribution of a novel pathway (ANAMMOX) in overall nitrogen removal. The ratio of NH4+-N (removed) & NO3--N (produced) in CW1 & CW2 indicated that ANAMMOX could be one of the key pathways for nitrogen removal in the CWs besides nitrification-denitrification in microbial films. The molecular analysis confirmed bands of ANAMMOX bacteria developed intrinsically. The study revealed that deep wetlands can offer a feasible option for the sustenance of ANAMMOX bacteria and may help develop design parameters for CWs for achieving higherT-N removal withsimilarsurface area as that of conventional wetlands.

Aluminium removal from water after defluoridation with the electrocoagulation process.

Fluoride is the most electronegative element and has a strong affinity for aluminium. Owing to this fact, most of the techniques used for fluoride removal utilized aluminium compounds, which results in high concentrations of aluminium in treated water. In the present paper, a new approach is presented to meet the WHO guideline for residual aluminium concentration as 0.2 mg/L. In the present work, the electrocoagulation (EC) process was used for fluoride removal. It was found that aluminium content in water increases with an increase in the energy input. Therefore, experiments were optimized for a minimum energy input to achieve the target value (0.7 mg/L) of fluoride in resultant water. These optimized sets were used for further investigations of aluminium control. The experimental investigations revealed that use of bentonite clay as coagulant in clariflocculation brings down the aluminium concentration of water below the WHO guideline. Bentonite dose of 2 g/L was found to be the best for efficient removal of aluminium.