Domestic wastewater sludge valorization: Multi-criteria evaluation of anaerobic digestion vs. hydrothermal liquefaction.

The emerging hydrothermal liquefaction (HTL) process is evaluated against the classical anaerobic digestion (AD) processes for stabilizing wastewater sludges and recovering their energy- and nutrient-contents. Although HTL affords faster stabilization, better process stability, and liquid fuel and sterile fertilizer recovery, it suffers from higher energy demand and lower technology readiness level. For a rational comparison of these pathways, a multi-criteria evaluation is conducted considering 21 technical, environmental, economic, and social criteria. Criteria values for the HTL-pathway were derived from laboratory tests while those for the AD-pathway were compiled from literature. Of the 16 process alternatives evaluated, the AD-pathway including nitrogen-recovery by air-stripping and phosphorus recovery by the MEPHREC® process ranked first followed by the HTL-pathway. This multi-criteria study suggests that the HTL-pathway could be engineered as a superior alternative for sludge stabilization and resource recovery if phosphorus recovery and its technology readiness level could be improved.

Feasibility of ammonium sulfate recovery from wastewater sludges: Hydrothermal liquefaction pathway vs. anaerobic digestion pathway.

This study evaluated two pathways to recover the nitrogen-content of wastewater sludges as ammonium sulfate (AmS) for use as fertilizer. The first pathway entails sludge stabilization by hydrothermal liquefaction (HTL) followed by recovery of AmS from the resulting aqueous product by gas permeable membrane (GPM) separation. The second one entails stabilization of the sludges by anaerobic digestion (AD) followed by recovery of AmS from the resulting centrate by GPM separation. A bench-scale GPM reactor is shown to be capable of recovering >90% of N in the feed. Recoveries of NH3-N in the HTL-pathway ranged 96-100% in 5.5-7.5 h at mass removal rates of 0.2-0.3 g N/day, yielding 3.3-6.0 g AmS/L of feed. Recoveries of 98% were noted in the AD-pathway in 4 h at mass removal rates of 0.06-0.97 g N/day and a yield of 1.7-2.1 g AmS/L of feed. Inductively coupled plasma optical emission spectrometer analysis confirmed that both pathways yielded AmS meeting the US EPA and European region guidelines for land application. The GPM reactor enabled higher nitrogen-recoveries in the HTL-pathway than those reported for current practice of AD followed by ammonia stripping, ion exchange, reverse osmosis, and/or struvite precipitation (96-100% vs. 50-90%). A process model for the GPM reactor is validated using performance data on three different feedstocks.

Biofertilizer recovery from organic solid wastes via hydrothermal liquefaction.

Hydrothermal liquefaction (HTL) has emerged as a viable pathway for processing wet organic solid wastes (OSW) to yield biocrude oil which could be upgraded to transportation fuels and specialty chemicals. The HTL process results in two byproducts laden with high levels of carbon, nitrogen, and phosphorous. Recovery of phosphates in the byproducts as struvite and ammoniacal-nitrogen (NH4-N) as ammonium sulfate is proposed here as a promising pathway to utilize the HTL byproducts. A case study of this pathway using algal biomass as a model OSW yielded 8.2 g struvite/100 g OSW and 10.7 g ammonium sulfate/100 g OSW. Heavy metal levels in both struvite and ammonium sulfate crystals were below EPA regulations for land application. This biofertilizer recovery pathway could render OSW processing by HTL a greener alternative to anaerobic digestion, offering feedstock versatility, substantially smaller footprint, and a higher degree of OSW valorization.

Techno-economic optimization of phosphorous recovery in an algal-based sewage treatment system.

Previous reports have documented the technical viability of an algal pathway for treating primary effluent and recovering its phosphorus-content (P) via hydrothermal liquefaction (HTL) of the resulting biomass. In this pathway, leaching P from HTL-derived biochar was found as the critical step impacting the economics of P-recovery. As such, a process model was developed in the current study to optimize P leaching from biochar as a function of five parameters. Model predictions under various conditions agreed well with measured data (r2 = 0.93; n = 184). The validated process model was then integrated with a cost model to establish the following conditions as optimal for leaching P from biochar: batch leaching time of 72 h; eluent NaOH concentration of 0.5 M; eluent-to-biochar ratio of 20; temperature of 60 °C; and provision of mixing. Under these conditions, 73.5% of P from biochar could be recovered at $5.98/kg P.

Removal and recovery of nutrients from municipal sewage: Algal vs. conventional approaches.

This paper presents a pilot scale study of an algal-based sewage treatment and resource recovery (STaRR) system capable of treating municipal sewage and recovering its nitrogen- and phosphorous-content as fertilizer. Core components of the STaRR system include i) mixotrophic cultivation of algal biomass in settled sewage; ii) hydrothermal liquefaction (HTL) of the resulting algal biomass, and iii) processing of the products of HTL to recover energy in the form of biocrude and nutrients in the form of struvite. Performance of a pilot-scale STaRR system in recovering nitrogen (N) and phosphorus (P) from settled sewage as struvite is documented and compared with that of existing and emerging technologies. Nutrient removal per unit energy input in the STaRR system is estimated as 257.1 g N/kWh and 36.6 g P/kWh while that in eight full-scale sewage treatment plants (STPs) averaged 74.3 g N/kWh and 135.1 g P/kWh. Energy required to treat primary effluent in the STaRR system (531.5 kWh/MG) is estimated to be lower than the average in the 8 STPs (1,037.9 ± 503.3 kWh/MG). While existing technologies had been originally designed for removal of nutrients rather than any recovery, a review of the literature revealed 12 emerging technologies for nutrient recovery. Nutrient recovery performance of the STaRR system (5.9% N and 71.6% P) is shown to be superior to that of those 12 emerging technologies. Recoveries recorded in the STaRR system translate to a yield of 2.4 kg struvite per 100 m3 of primary effluent. Results of this study imply that the STaRR system deserves due consideration as a greener and sustainable pathway for nutrient removal and recovery from sewage.

Algal-based, single-step treatment of urban wastewaters.

Currently, urban wastewaters (UWW) laden with organic carbon (BOD) and nutrients (ammoniacal nitrogen, N, and phosphates, P) are treated in multi-stage, energy-intensive process trains to meet the mandated discharge standards. This study presents a single-step process based on mixotrophic metabolism for simultaneous removal of carbon and nutrients from UWWs. The proposed system is designed specifically for hot, arid environments utilizing an acidophilic, thermotolerant algal species, Galdieria sulphuraria, and an enclosed photobioreactor to limit evaporation. Removal rates of BOD, N, and P recorded in this study (14.93, 7.23, and 1.38 mg L(-1) d(-1), respectively) are comparable to literature reports. These results confirm that the mixotrophic system can reduce the energy costs associated with oxygen supply in current UWW treatment systems, and has the potential to generate more energy-rich biomass for net energy extraction from UWW.

Algal biofuels from urban wastewaters: maximizing biomass yield using nutrients recycled from hydrothermal processing of biomass.

Recent studies have proposed algal cultivation in urban wastewaters for the dual purpose of waste treatment and bioenergy production from the resulting biomass. This study proposes an enhancement to this approach that integrates cultivation of an acidophilic strain, Galdieria sulphuraria 5587.1, in a closed photobioreactor (PBR); hydrothermal liquefaction (HTL) of the wet algal biomass; and recirculation of the nutrient-rich aqueous product (AP) of HTL to the PBR to achieve higher biomass productivity than that could be achieved with raw wastewater. The premise is that recycling nutrients in the AP can maintain optimal C, N and P levels in the PBR to maximize biomass growth to increase energy returns. Growth studies on the test species validated growth on AP derived from HTL at temperatures from 180 to 300°C. Doubling N and P concentrations over normal levels in wastewater resulted in biomass productivity gains of 20-25% while N and P removal rates also doubled.

Evaluation of a thermo-tolerant acidophilic alga, Galdieria sulphuraria, for nutrient removal from urban wastewaters.

Nutrient removal from primary wastewater effluent was tested using Galdieria sulphuraria, an acidophilic and moderately thermophilic alga. Biomass yield recorded in this study (27.42g biomass per g nitrogen removed) is higher than the average reported in the literature (25.75g g(-1)) while, the theoretical yield estimated from the empirical molecular formula of algal biomass is 15.8g g(-1). Seven-day removal efficiencies were 88.3% for ammoniacal-nitrogen and 95.5% for phosphates; corresponding removal rates were 4.85 and 1.21mg L(-1)d(-1). Although these rates are lower than the average literature values for other strains (6.36 and 1.34mg L(-1)d(-1), respectively), potential advantages of G. sulphuraria for accomplishing energy-positive nutrient removal are highlighted. Feasibility of growing G. sulphuraria outdoors at densities higher than in high-rate oxidation ponds is also demonstrated.

Low energy/cost desalination: low dose and low mean ion resident time in concentrate stream of electro-dialysis reversal.

Species, dose, and mean ion resident time (MIRT(c)) in the concentrate of electro-dialysis reversal (EDR) desalination are analysed. In the classical EDR, dimensions, flow, and velocity of dilute and concentrate are equal; Langelier saturation index (LSI) and CaSO4 saturation are used to control the scaling and fouling processes in concentrate, as such LSI < + 2.16 for preventing CaCO3 from fouling and CaSO4 saturation level < 200 for averting CaSO4 from precipitation. If LSI is more than allowable limit, acid is added in concentrate to keep CaCO3 continuously dissolving; if CaSO4 saturation level in concentrate is more than the allowable limit, sodium hexametaphosphate (SHMP) is added in concentrate to maintain CaSO4 dissolving. EDR, however, was successfully modernised to operate with the higher water recovery rate (R) without any anti-scalant and without acid; this new EDR operated with LSI at 2.29 and CaSO4 saturation level 358.9% at lower dose and lower MIRT(c). Dose and MIRT(c) are proposed to address the controlling process. Monographs for the acids and SHMP requirements, and for the desalting cost including desalting power, membrane surface area, and chemicals usage, are developed. By lowering R and polar reversal interval, EDR can be operated at MIRT(c) < 130 min; at MIRT(c0 < 130 min, desalting cost/ energy can be minimised by eliminating chemicals requirement.

Cattle wastes as substrates for bioelectricity production via microbial fuel cells.

A new 2 l scale microbial fuel cell (MFC) configuration was developed to generate bioelectricity from particulate substrates. Voltage and power densities generated in this MFC fed with sucrose, particulate cattle manure, and manure wash-water as the substrates were evaluated in batch mode, with and without external mediators. Voltages averaged 0.5 V in open circuit, and 0.4 V under a resistive load of 470 Ω. Power densities (67 to 215 mW/m(2)) were comparable to previous work that had used liquid wastes as substrates. Based on the energy yield per unit mass of feedstock (~10 kJ/kg wet manure), cattle manure has limited potential to serve as a feedstock for electricity production via MFCs.

Enhancing anaerobic hydrolysis of cattle manure in leachbed reactors.

In this study, it is hypothesized that anaerobic hydrolysis of organic solid wastes (OSWs) in leachbed reactors can be enhanced by two approaches: (1) by increasing the porosity of the leachbed; and (2) by seeding the bed with brown-rot fungi or anaerobically digested residues. The hypothesis is verified using cattle manure as a model OSW and pistachios-half-shell as an inert additive to increase the porosity of the leachbed. The enhancement was quantified in terms of volatile fatty acid (VFA) generation and soluble COD generation. Results of this study showed that 15% more VFAs and 8% more soluble COD can be obtained at higher bed porosities and with brown-rot fungi or digested residues as the seed. Compared to a leachbed without any recirculation, porosity enhancement, nor seed addition, the VFA yield (g VFA/g manure) in a leachbed with pistachios-half-shell as porosity enhancers was 193% higher; that in a leachbed with leachate recirculation (of 4 L/day), pistachios-half-shell as porosity enhancers, and brown-rot fungi or digested sludge as seed was 230% higher.

Anaerobic fermentation of cattle manure: modeling of hydrolysis and acidogenesis.

A mathematical model for the hydrolysis and acidogenesis reactions in anaerobic digestion of cattle manure is presented. This model is based on the premise that particulate hydrolysable fraction of cattle manure is composed of cellulose and hemicellulose that are hydrolyzed at different rates according to a surface-limiting reaction; and, that the respective soluble products of hydrolysis are utilized by acidogens at different rates, according to a two-substrate, single-biomass model. Batch experimental results were used to identify the sensitive parameters and to calibrate and validate the model. Results predicted by the model agreed well with the experimentally measured data not used in the calibration process, with correlation coefficient exceeding 0.91. These results indicate that the most significant parameter in the hydrolysis-acidogenesis phase is the hydrolysis rate constant for the cellulose fraction.

Computer-aided tutorials and tests for use in distance learning.

Compared to the traditional on-campus students, remote students in distance education courses find it more difficult to develop problem-solving skills. In this paper, we propose the use of computer-aided tutorials and tests (CATTs) as convenient tools for remote students to practice and improve problem-solving and test taking skills at their own pace. Example of CATTs developed with Authorware software for use in distance education is presented.

Evaluation of an ozonation system for reduced waste sludge generation.

The ultimate disposal of biosolids has been and continues to be one of the most expensive problems faced by wastewater utilities. Previous work has shown that the waste sludge generation in an activated sludge plant can be reduced by promoting cryptic growth conditions (i.e., biomass growth on intracellular products). For this purpose, excess biosolids from a continuous flow activated sludge system were solubilized using ozone as the cell lysing agent, and then returned to the aeration tank. The results of these preliminary studies indicate that the proposed process configuration has the potential to reduce the waste sludge production by 40% to 60%. In the present research, the details of the ozonation process is further investigated to determine the maximum solubilization efficiency. For this purpose, a number of variables such as the solids concentration in the excess sludge, ozonation time, and ozonation dosage rate are studied.

Microbial toxicity in soil medium.

A laboratory procedure using the respirometer was developed to measure microbial toxicity in soils of organic chemicals. The procedure was tested with Polytox, a commercial microbial test culture, on 35 organic chemicals. The test protocol was reproducible with a mean standard deviation of 0.08 mg chemical/g of soil for eight chemicals. The assays were also carried out at different moisture-holding capacities of soil. Correlation between toxicity tests done in aqueous medium and soil for Polytox test culture was statistically significant and yielded an r2 of 0.816. Details of the laboratory procedure and test results are presented together with comparisons of toxicity of chemicals in the soil medium with that in aqueous medium for the same test organism. A few practical applications are discussed.

Structure- and property-activity relationship models for prediction of microbial toxicity of organic chemicals to activated sludge.

Two quantitative structure-activity relationship (QSAR) and two quantitative property-activity relationship (QPAR) models reported in the literature for predicting toxicity of synthetic organic chemicals to activated sludge microorganisms are summarized and compared. The QSAR models were developed using solvatochromic parameters and molecular connectivity indices; the QPAR models, using octanol-water partition coefficient and aqueous solubility. Experimental data on 16 chemicals not used in developing the above models are used to compare and evaluate the predictive ability of these QSAR/QPAR models. Based on the quality of the original models, their predictions, ease of application, and availability of model parameters, molecular connectivity indices and log P appear to be the most suitable in toxicity predictions.

Additivity in microbial toxicity of nonuniform mixtures of organic chemicals.

Microbial toxicity of nonuniform mixtures of selected synthetic organic chemicals in several proportion is evaluated. Toxicity is quantified by the inhibition of oxygen uptake rate of a surrogate microbial text culture as measured by a respirometer. The joint toxic effects of the chemicals are analyzed for simple addition using toxic units (TU) and similarity parameters (lambda). A new approach is proposed to assign acceptance limits to sigma(TU)i and lambda to account for experiment errors and variances. Based on this approach, the joint toxic effects of 16 chemicals evaluated in this study in 14 different mixtures were found to be simply additive. Predictions of component concentration based on simple additivity agreed with the measured values within an average factor of error of 1.4.