Quantitative assessment of energy and resource recovery in wastewater treatment plants based on plant-wide simulations.

The growing development of technologies and processes for resource treatment and recovery is offering endless possibilities for creating new plant-wide configurations or modifying existing ones. However, the configurations' complexity, the interrelation between technologies and the influent characteristics turn decision-making into a complex or unobvious process. In this frame, the Plant-Wide Modelling (PWM) library presented in this paper allows a thorough, comprehensive and refined analysis of different plant configurations that are basic aspects in decision-making from an energy and resource recovery perspective. In order to demonstrate the potential of the library and the need to run simulation analyses, this paper carries out a comparative analysis of WWTPs, from a techno-economic point of view. The selected layouts were (1) a conventional WWTP based on a modified version of the Benchmark Simulation Model No. 2, (2) an upgraded or retrofitted WWTP, and (3) a new Wastewater Resource Recovery Facilities (WRRF) concept denominated as C/N/P decoupling WWTP. The study was based on a preliminary analysis of the organic matter and nutrient energy use and recovery options, a comprehensive mass and energy flux distribution analysis in each configuration in order to compare and identify areas for improvement, and a cost analysis of each plant for different influent COD/TN/TP ratios. Analysing the plants from a standpoint of resources and energy utilization, a low utilization of the energy content of the components could be observed in all configurations. In the conventional plant, the COD used to produce biogas was around 29%, the upgraded plant was around 36%, and 34% in the C/N/P decoupling WWTP. With regard to the self-sufficiency of plants, achieving self-sufficiency was not possible in the conventional plant, in the upgraded plant it depended on the influent C/N ratio, and in the C/N/P decoupling WWTP layout self-sufficiency was feasible for almost all influents, especially at high COD concentrations. The plant layouts proposed in this paper are just a sample of the possibilities offered by current technologies. Even so, the library presented here is generic and can be used to construct any other plant layout, provided that a model is available.

Optimization of a thermal hydrolysis process for sludge pre-treatment.

At industrial scale, thermal hydrolysis is the most used process to enhance biodegradability of the sludge produced in wastewater treatment plants. Through statistically guided Box-Behnken experimental design, the present study analyses the effect of TH as pre-treatment applied to activated sludge. The selected process variables were temperature (130-180 °C), time (5-50 min) and decompression mode (slow or steam-explosion effect), and the parameters evaluated were sludge solubilisation and methane production by anaerobic digestion. A quadratic polynomial model was generated to compare the process performance for the 15 different combinations of operation conditions by modifying the process variables evaluated. The statistical analysis performed exhibited that methane production and solubility were significantly affected by pre-treatment time and temperature. During high intensity pre-treatment (high temperature and long times), the solubility increased sharply while the methane production exhibited the opposite behaviour, indicating the formation of some soluble but non-biodegradable materials. Therefore, solubilisation is not a reliable parameter to quantify the efficiency of a thermal hydrolysis pre-treatment, since it is not directly related to methane production. Based on the operational parameters optimization, the estimated optimal thermal hydrolysis conditions to enhance of sewage sludge digestion were: 140-170 °C heating temperature, 5-35min residence time, and one sudden decompression.

A novel configuration for an anaerobic submerged membrane bioreactor (AnSMBR). Long-term treatment of municipal wastewater under psychrophilic conditions.

A novel design for a pilot scale anaerobic submerged membrane bioreactor (AnSMBR) equipped with an ultrafiltration unit, treating municipal wastewater at 18 ± 2°C, and inoculated with a mesophilic inoculum without acclimation, was implemented and evaluated over 3 years of stable operation. The AnSMBR operated with a volumetric loading rate between 1.6 to 2.0 kg COD/m(3)UASBd, 12.8 to 14.2h hydraulic retention time, and reached a tCOD removal efficiency of around 90%. Biosolid production was between 0.05 and 0.083 g VS/g CODremoved. Dissolved methane oversaturation in the effluent was observed, reaching average values of 19.1 ± 0.84 mg CH4/L. The permeate flow rate ranged from 10 to 14L/m(2)h with trans-membrane pressure (TMP) values of 400-550 mbar, using cycles of 30s backwash, 7.5 min filtration, and continuous biogas sparging (9-16 m/h). During the three years of continuous operation, the membrane was not physically or chemically cleaned.

Evaluation of thermal steam-explosion key operation factors to optimize biogas production from biological sludge.

Thermal steam-explosion is the most extended hydrolysis pretreatment to enhance anaerobic digestion of sludge. Thermal hydrolysis key parameters are temperature (T) and time (t), and the generally accepted values reported from full-scale information are: 150-230 °C and 20-60 min. This study assesses the influence of different temperature-time-flash combinations (110-180 °C, 5-60 min, 1-3 re-flashing) on the anaerobic degradation of secondary sludge through biochemical methane potential (BMP) tests. All the conditions tested presented higher methane production compared to the untreated sludge, and both solubilization (after the hydrolysis) and degradation (by anaerobic digestion) increased linearly when increasing the severity (T-t) of the pretreatment, reaching 40% solubilization and degradation of the particulate matter at 180° C-60 min. However, for the 180 °C temperature, the treatment time impacted negatively on the lag phase. No influence of re-flashing the pretreated matter was observed. In conclusion, thermal steam-explosion at short operation times (5 min) and moderate temperatures (145 °C) seems to be very attractive from a degradation point of view thus presenting a methane production enhancement similar to the one obtained at 180°C and without negative influence of the lag phase.

Long-term operation of a pilot scale anaerobic membrane bioreactor (AnMBR) for the treatment of municipal wastewater under psychrophilic conditions.

The performance of a pilot scale anaerobic membrane bioreactor (AnMBR), comprising an upflow anaerobic sludge blanket (UASB) reactor coupled to an external ultrafiltration membrane treating municipal wastewater at 18±2°C, was evaluated over three years of stable operation. The reactor was inoculated with a mesophilic inoculum without acclimation. The AnMBR supported a tCOD removal efficiency of 87±1% at hydraulic retention time (HRT) of 7h, operating at a volumetric loading rate (VLR) of between 2 and 2.5kgtCOD/m(3)d, reaching effluent tCOD concentrations of 100-120mg/L and BOD5 concentrations of 35-50mgO2/L. Specific methane yield varied from 0.18 to 0.23Nm(3)CH4/kgCODremoved depending on the recirculation between the membrane module and the UASB reactor. The permeate flow rate, using cycles of 15s backwash, 7.5min filtration, and continuous biogas sparging (40-60m/h), ranged from 10 to 14Lm(2)/h with trans-membrane pressure (TMP) values of 400-550mbar.

A feasibility study on the bioconversion of CO2 and H2 to biomethane by gas sparging through polymeric membranes.

In this study, the potential of a pilot hollow-fiber membrane bioreactor for the conversion of H2 and CO2 to CH4 was evaluated. The system transformed 95% of H2 and CO2 fed at a maximum loading rate of 40.2 [Formula: see text] and produced 0.22m(3) of CH4 per m(3) of H2 fed at thermophilic conditions. H2 mass transfer to the liquid phase was identified as the limiting step for the conversion, and kLa values of 430h(-1) were reached in the bioreactor by sparging gas through the membrane module. A simulation showed that the bioreactor could upgrade biogas at a rate of 25m(3)/mR(3)d, increasing the CH4 concentration from 60 to 95%v. This proof-of-concept study verified that gas sparging through a membrane module can efficiently transfer H2 from gas to liquid phase and that the conversion of H2 and CO2 to biomethane is feasible on a pilot scale at noteworthy load rates.

Effect of thermal hydrolysis and ultrasounds pretreatments on foaming in anaerobic digesters.

Foam appears regularly in anaerobic digesters producing operational and safety problems. In this research, based on the operational observation at semi-industrial pilot scale where sludge pretreatment mitigated foaming in anaerobic digesters, this study aimed at evaluating any potential relationship between foaming tools applied to activated sludge at lab-scale (foam potential, foam stability and Microthrix parvicella abundance) and the experimental behavior observed in pilot scale and full-scale anaerobic digesters. The potential of thermal hydrolysis and ultrasounds for reducing foaming capacity was also evaluated. Filamentous bacteria abundance was directly linked to foaming capacity in anaerobic processes. A maximum reduction of M.parvicella abundance (from 5 to 2) was reached using thermal hydrolysis with steam explosion at 170°C and ultrasounds at 66.7kWh/m(3), showing both good anti-foaming properties. On the other hand, foam potential and stability determinations showed a lack of consistency with the bacteria abundance results and experimental evidences.

Thermal steam explosion pretreatment to enhance anaerobic biodegradability of the solid fraction of pig manure.

The assessment of the biodegradability of thermal steam-exploded pig manure was performed compared to untreated samples. The pre-treatment was performed under different combinations of temperature and time, ranging 150-180 °C and 5-60 min, and used as substrate in a series of batch biochemical methane potential (BMP) tests. Results were analyzed in terms of methane yield, kinetic parameters and severity factor. In all the pre-treatment conditions, methane yield and degradation rates increased when compared to untreated pig slurry. An ANOVA study determined that temperature was the main factor, and the optimum combination of temperature-time of pretreatment was 170 °C -30 min, doubling methane production from 159 to 329 mL CH4/gVSfed. These operation conditions correspond to a severity factor of 3.54, which was considered an upper limit for the pretreatment due to the possible formation of inhibitory compounds, hindering the process if this limit is exceeded.

Influence of thermal pretreatment on the biochemical methane potential of wheat straw.

The biochemical methane potential of steam exploded wheat straw was evaluated in a pilot plant under different temperature-time combinations. The optimum was obtained for 1 min and 220 °C thermal pretreatment (3.5 severity factor), resulting in a 20% increase in methane production respect non-treated straw. For more severe treatments the biodegradability decreased due to a possible formation of inhibitory compounds. The results of the tests were modeled with a first order equation to estimate the hydrolysis constant and biodegradability extent, and the influence of temperature and time on the kinetic parameters was obtained with a response surface study. The data processing confirmed the accuracy of the model and the optimum operation conditions, and demonstrated that the biomethanization of raw and pretreated wheat straw is limited by the hydrolysis, being the individual influence of temperature and time much more important than the interaction between them.

Working with energy and mass balances: a conceptual framework to understand the limits of municipal wastewater treatment.

At present all municipal waste water treatment plants (WWTPs) are energy consumers. Electrical energy requirements for oxygen transfer are large in secondary biological systems. Nevertheless, from a thermodynamic point of view chemical oxygen demand (COD) is an energy source. Combustion of every kilogram of COD releases 3.86 kWh of energy. In this manuscript some measures are presented, from a conceptual point of view, in order to convert the actual concept of wastewater treatment as an 'energy sink' to an 'energy source' concept. In this sense, electrical self-sufficiency in carbon removal WWTPs could be obtained by increasing the sludge load to the anaerobic sludge digester. Nitrogen removal increases the energy requirements of WWTPs. The use of a combined two-stage biological treatment, using a high loaded first stage for carbon removal and a second stage combined nitrification-anammox process for nitrogen removal in the water line, offers a way to recover self-sufficiency. This is not a proven technology at ambient temperature, but its development offers an opportunity to reduce the energy demand of WWTPs.

Biochemical methane potential of microalgae: influence of substrate to inoculum ratio, biomass concentration and pretreatment.

The anaerobic digestion of three microalgae mixtures was evaluated at different substrate to inoculum (S/I) ratios (0.5, 1 and 3), biomass concentrations (3, 10 and 20gTS/kg) and pretreatments (thermal hydrolysis, ultrasound and biological treatment). An S/I ratio of 0.5 and 10gTS/kg resulted in the highest final methane productivities regardless of the microalgae tested (ranging from 188 to 395mL CH(4)/gVS(added)). The biological pretreatment supported negligible enhancements on CH(4) productivity, while the highest increase (46-62%) was achieved for the thermal hydrolysis. The optimum temperature of this pretreatment depended on the microalgae species. The ultrasound pretreatment brought about increases in CH(4) productivity ranging from 6% to 24% at 10,000kJ/kgTS, without further increases at higher energy inputs. The results here obtained confirmed the lack of correlation between the solubilization degree and the methane enhancement potential and pointed out that anaerobic digestion of algae after thermal pretreatment is a promising technology for renewable energy production.

Continuous thermal hydrolysis and anaerobic digestion of sludge. Energy integration study.

Experimental data obtained from the operation in a pilot plant are used to perform mass and energy balances to a global process combining units of thermal hydrolysis (TH) of secondary sludge, anaerobic digestion (AD) of hydrolysed secondary sludge together with fresh primary sludge, and cogeneration from biogas by using a gas engine in which the biogas produces electricity and heat from the exhaust gases. Three scenarios were compared, corresponding to the three digesters operated: C (conventional AD, 17 days residence time), B (combined TH + AD, same time), and A (TH + AD at half residence time). The biogas production of digesters B and A was 33 and 24% better, respectively when compared with C. In the case of the combined TH + AD process (scenarios A and B), the key factors in the energy balance were the recovery of heat from hot streams, and the concentration of sludge. The results of the balances showed that for 8% DS concentration of the secondary sludge tested in the pilot plant, the process can be energetically self-sufficient, but a fraction of the biogas must by-pass the gas engine to be directly burned. From an economic point of view, scenario B is more profitable in terms of green energy and higher waste removal, while scenario A reduces the digester volume required by a half. Considering a population of 100,000 inhabitants, the economic benefit is 87,600 €/yr for scenario A and 132,373 €/yr for B. This value can be increased to 223,867 €/yr by increasing the sludge concentration of the feeding to the TH unit to a minimum value that allows use of all the biogas to produce green energy. This concentration is 13% DS, which is still possible from a practical point of view. Additional benefits gained with the combined TH + AD process are the enhancement of the digesters rheology and the possibility of getting Class A biosolids. The integration study presented here set the basis for the scale-up to a demonstration plant.

Enhancement of the conventional anaerobic digestion of sludge: comparison of four different strategies.

Anaerobic digestion (AD) is the preferred option to stabilize sludge. However, the rate limiting step of solids hydrolysis makes it worth modifing the conventional mesophilic AD in order to increase the performance of the digester. The main strategies are to introduce a hydrolysis pre-treatment, or to modify the digestion temperature. Among the different pre-treatment alternatives, the thermal hydrolysis (TH) at 170 degrees C for 30 min, and the ultrasounds pre-treatment (US) at 30 kJ/kg TS were selected for the research, while for the non-conventional anaerobic digestion, the thermophilic (TAD) and the two-stage temperature phased AD (TPAD) were considered. Four pilot plants were operated, with the same configuration and size of anaerobic digester (200 L, continuously fed). The biogas results show a general increase compared to the conventional digestion, being the highest production per unit of digester for the process combining the thermal pre-treatment and AD (1.4 L biogas/L digester day compared to the value of 0.26 obtained in conventional digesters). The dewaterability of the digestate became enhanced for processes TH + AD and TPAD when compared with the conventional digestate, while it became worse for processes US + AD and TAD. In all the research lines, the viscosity in the digester was smaller compared to the conventional (which is a key factor for process performance and economics), and both thermal pre-treatment and thermophilic digestion (TAD and TPAD) assure a pathogen free digestate.

Assessment of the influence of thermal pre-treatment time on the macromolecular composition and anaerobic biodegradability of sewage sludge.

Laboratory and pilot-scale experiments were carried out in order to evaluate the influence of thermal pre-treatment time on waste-activated sludge properties and anaerobic biodegradability. Six experimental conditions were analyzed from 0 to 30 min of hydrolysis time. Solubilization of macromolecular compounds, changes in the main sludge properties and anaerobic biodegradability of the sewage sludge were evaluated. A similar carbohydrate solubilization degree was achieved, from 53% to 70% and 59% to 75% for lab- and pilot-scale experiments, respectively. In the case of proteins, the values of solubilization were lower in the pilot-scale experiment than in the laboratory, with 31-45% and 47-70%, respectively. Ammonia and volatile fatty acid did not undergo important changes; however the sludge dewaterability enhanced at increased pre-treatment times. All the pre-treatment conditions had a positive effect with regard to anaerobic biodegradability and by fitting experimental data with a simplified mathematical model, it was concluded that the maximum biogas production rate is more influenced by the pre-treatment time than the total biogas production.

Full-stream and part-stream ultrasound treatment effect on sludge anaerobic digestion.

The use of ultrasound as pre-treatment to improve anaerobic digestion of secondary sludge has been established as a promising technology. There are great differences between lab scale and full-scale devices, regarding the relationship between the disintegration achieved and the energy supplied. Based on economic aspects, most of the full-scale plants use partial-stream instead of the full-stream sonication, which affects biogas production and digestate dewatering characteristics. A laboratory scale operation combining ultrasound and anaerobic digestion (batch tests) has been performed, determining the relationship between the ratio of sonicated sludge fed and the methane production, SCOD removal and capillary suction time after 20-day anaerobic biodegradation, in order to check the possible benefits of part-stream versus full-stream sonication. Additional incubation was also evaluated, searching for an optimum process combining ultrasound and 24-h incubation pretreatment. Results showed that by sonicating fresh WAS at 25,700 kJ/kg TS biogas yield increased linearly with the percentage of sonicated WAS in the substrate, from 248 (control reactor) to 349 mL CH(4)/g VS (41% increase in full-stream sonication). By incubation (24 h, 55 degrees C), 325 mL CH(4)/g VS were obtained (31% increase), but the digestion of the soluble compounds generated during incubation of sonicated sludge appeared to be less degradable compared to those solubilised by ultrasound or incubation alone, which showed no benefit in combining both treatments. Post-digestion dewatering deteriorated for both part-stream and full-stream sonication, and CST values were constant (74% higher than the control digestate) from 30% to 100% sonicated sludge.

Hydrogen sulphide removal in the anaerobic digestion of sludge by micro-aerobic processes: pilot plant experience.

H(2)S removal from biogas produced in anaerobic digestion of sludge through the introduction of oxygen under micro-aerobic conditions is studied. Research was carried out in two pilot plant reactors (working volume, 200 L each) treating sludge from WWTP with HRT of 20 days. Mixing was provided via sludge or biogas recirculation. Introduction of very low oxygen flow (0.013-0.024 L/L(reactor) d) successfully removed H(2)S content in biogas with an efficiency above 99%. Reactor performance during micro-aerobic operation in terms of biogas production, methane yield and COD removal were not affected by the amount of oxygen supplied, remaining stable and similar to the anaerobic behaviour. Sludge recirculation ( approximately 50 L/h) and biogas recirculation ( approximately 3.5 L/min) as mixing methods were found not significant in H(2)S removal from biogas while biogas recirculation reduced by 10 times dissolved sulphide concentration compared to sludge recirculation.

Ultrasound pre-treatment for anaerobic digestion improvement.

Prior research indicates that ultrasounds can be used in batch reactors as pre-treatment before anaerobic digestion, but the specific energy required at laboratory-scale is too high. This work evaluates both the continuous ultrasound device performance (efficiency and solubilisation) and the operation of anaerobic digesters continuously fed with sonicated sludge, and presents energy balance considerations. The results of sludge solubilisation after the sonication treatment indicate that, applying identical specific energy, it is better to increase the power than the residence time. Working with secondary sludge, batch biodegradability tests show that by applying 30 kWh/m3 of sludge, it is possible to increase biogas production by 42%. Data from continuous pilot-scale anaerobic reactors (V=100 L) indicate that operating with a conventional HRT=20 d, a reactor fed with pre-treated sludge increases the volatile solids removal and the biogas production by 25 and 37% respectively. Operating with HRT=15 d, the removal efficiency is similar to the obtained with a reactor fed with non-hydrolysed sludge at HTR=20 d, although the specific biogas productivity per volume of reactor is higher for the pretreated sludge. Regarding the energy balance, although for laboratory-scale devices it is negative, full-scale suppliers state a net generation of 3-10 kW per kW of energy used.

Continuous thermal hydrolysis and energy integration in sludge anaerobic digestion plants.

A thermal hydrolysis pilot plant with direct steam injection heating was designed and constructed. In a first period the equipment was operated in batch to verify the effect of sludge type, pressure and temperature, residence time and solids concentration. Optimal operation conditions were reached for secondary sludge at 170 degrees C, 7 bar and 30 minutes residence time, obtaining a disintegration factor higher than 10, methane production increase by 50% and easy centrifugation In a second period the pilot plant was operated working with continuous feed, testing the efficiency by using two continuous anaerobic digester operating in the mesophilic and thermophilic range. Working at 12 days residence time, biogas production increases by 40-50%. Integrating the energy transfer it is possible to design a self-sufficient system that takes advantage of this methane increase to produce 40% more electric energy.

Sulfide removal by moderate oxygenation of anaerobic sludge environments.

Introduction of a limited amount of oxygen to anaerobic bioreactors is proposed as a simple technique to lower the level of sulfide in the biogas. This paper presents the results of a bioreactor study and of batch experiments that were performed to obtain better insight into the fate of sulfur compounds and oxygen during micro-aerobic sulfide oxidation. Introduction of a low airflow (0.7-0.9 m(3)m(-3)d(-1), corresponding to an O(2)/S molar ratio of 8-10) to a fluidized bed reactor fed with low-sulfate vinasse was sufficient to reduce the biogas H(2)S-content to an undetectable level. Sulfide was initially oxidized to elemental sulfur, thiosulfate and - most probably - polysulfide. Significant sulfate production did not occur. Bioreactor sludge sampled from the reactor after three weeks' micro-aerobic operation was much faster in oxidizing sulfur than bioreactor sludge sampled during fully anaerobic reactor operation. The reaction proceeded faster with increasing O(2)/sulfide ratios.

Automated manometric method to assess anaerobic toxicity of chemicals.

Industrial additives eventually used for different purposes (antifoaming, cleaning, bactericides, antiscale, etc) are discharged to the wastewater treatment plant. The anaerobic toxicity of these commercial products is not provided by suppliers. A new manometric method is developed and tested to evaluate anaerobic toxicity or inhibition using four different commercial products. Antifoaming Cleron 6 (50-200 ppm), bactericide Divosan-forte (0.05-1.0% v/v), bleach (0.1-1.0% v/v) and cleaning agent Topax 66 (0.10-1.0% v/v). According to the different methods proposed in the literature, from the methane production rate, it is possible to calculate both methanogenic activity evolution and final substrate removal and quantify the potential inhibitory effect of commercial additives. The experimental method is simple and reliable.