Nitrogen removal in domestic wastewater. Effect of nitrate recycling and COD/N ratio.

A denitrification/nitrification pilot plant was designed, built and put into operation, treating the effluent of an anaerobic reactor. The operation of the plant examined the effect of the nitrate recycling and the COD/N ratio on the nitrogen and the remaining organic matter removal at 18 °C. The system consisted of a two-stage treatment process: anoxic and aerobic. The hydraulic retention time (HRT) of the system was 1 h for the anoxic bioreactor and 2 h for the aerobic one. The increase in the nitrate recycling ratio did not cause a significant improvement in the nitrogen removal due to the insufficient carbon source. The wastewater to be treated had a C/N ratio of 1.1 showing a lack of organic carbon. The addition of methanol was a key point in the denitrification process used as a model for the traditional wastewater by-pass in the WWTP. The maximum nitrogen and organic matter removal (87.1% and 96%, respectively) was achieved with a nitrate recycling ratio of 600% and a C/N of 8.25, adjusted by methanol addition.

Sequencing batch reactor process for the removal of nitrogen from anaerobically treated domestic wastewater.

This work presents the performance of a sequencing batch reactor (SBR) system used as a means of removing nitrogen from domestic wastewater containing a low chemical oxygen demand (COD) to nitrogen ratio due to pre-treatment with an anaerobic reactor. The aim of the work was to determine the feasibility of this system for the removal of nitrogen from the domestic wastewater. An SBR with a working volume of 5 L was investigated at different cycle times of 12, 8 and 6 h, at 18 °C. The efficiency of the SBR varied together with the duration of the cycle, where the optimum performance was seen in the 6 h cycle with the anoxic-aerobic-anoxic sequence. Due to the low quantity of organic matter present in the domestic wastewater after the anaerobic treatment, an additional supply of external carbon was necessary before the second anoxic stage. The removal efficiencies obtained were: 98% for total Kjeldahl nitrogen, 84% for total nitrogen and 77% for soluble COD. The reactor was thus shown to be viable, and it was concluded that this process may be successfully applied as a post-treatment for the removal of nitrogen from anaerobically treated domestic wastewater.

Recirculation of gas emissions to achieve advanced denitrification of the effluent from the anaerobic treatment of domestic wastewater.

A denitrifying pilot plant was designed, constructed and operated for more than five months. The plant treated domestic wastewater with high ammonium nitrogen concentration, which had previously undergone an anaerobic process at 18 °C. The process consisted of one biofilter with 2 h of hydraulic retention time for denitritation. Different synthetic nitrite concentrations were supplied to the anoxic reactor to simulate the effluent of a nitritation process. This work investigates the advanced denitritation of wastewater using the organic matter and other alternative electron donors present in an anaerobic treatment process effluent: methane and sulfide. The denitrifying bacteria were able to treat wastewater at an inlet nitrite concentration of 75 mg NO2--N/L with a removal efficiency of 92.9%. When the inlet nitrite concentration was higher, the recirculation of the gas from the top of the anoxic reactor was successful to enhance the nitrite removal, achieving a NO2- elimination efficiency of 98.3%.

Denitrification of the anaerobic membrane bioreactor (AnMBR) effluent with alternative electron donors in domestic wastewater treatment.

A fixed film bioreactor for the denitrification of the effluent from an anaerobic membrane bioreactor (AnMBR) treating domestic wastewater was designed, built and investigated. After anaerobic treatment, the wastewater usually has a low C/N ratio (∼1.3), and a remaining chemical oxygen demand of around 117mg O2/L, which is not enough to make conventional heterotrophic denitrification possible. That effluent also holds methane and sulfide dissolved and oversaturated after leaving the AnMBR. This paper demonstrates the feasibility of using these reduced compounds as electron donors in order to remove 80mg NOx--N/L at 18°C and 2h of hydraulic retention time. In addition, the influence of the NO2-/NO3- ratios in the feed was studied. Total nitrogen removal was achieved in all the cases studied, except for a feed with 100% NO3-. Methane was the main electron donor used to remove the nitrites and nitrates, with a participation rate of over 70%.

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.

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.

Influence of velocity gradient in a hydraulic flocculator on NOM removal by aerated spiral-wound ultrafiltration membranes (ASWUF).

A hydraulic coagulation-flocculation processes combined with aerated spiral-wound ultrafiltration membranes (ASWUF) was designed with the objective of improving natural organic matter (NOM) removal by ASWUF in the treatment of water for human consumption. The pilot-scale experimental system had capacity for treating 0.9 m(3)/h. Dosage of Cl(3)Fe as coagulant and hydraulic retention time (HRT) were calculated to generate microflocculation and different velocity gradients (G=27, 47, 87 and 104 s(-1)) were applied in the hydraulic flocculator. Operating alone, the ASWUF system achieved an NOM removal performance of 39% without problems of membrane clogging, although there was a significant correlation between effluent and influent quality. Application of microflocculation achieved considerable improvement in NOM removal, but values of G< or =87 s(-1) resulted in rapid clogging of the membrane due to flocs disintegration in the aerated membrane tank. Particle analysis revealed that the reduction of the velocity gradient had the effect of inclining the particle size distribution towards larger sizes, affecting both NOM removal capacity and membrane clogging. For G=104 s(-1) an NOM removal yield of 90% was reached, while transmembrane pressure (TMP) was stabilised as a result of the control of membrane clogging.

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.

Potabilization of low NOM reservoir water by ultrafiltration spiral wound membranes.

Membrane technologies such as ultrafiltration offer an interesting alternative to integral treatment of surface water destined for human consumption. With this in mind, a pilot-scale ultrafiltration module was set up, equipped with spiral-wound polyethersulphone membranes (16.6m(2)) with an effective pore size of 0.05 microm. The system operated continuously with a stable production of 0.9 m(3)/h (54 lmh) and a constant transmembrane pressure of -0.2 bar. The effluent obtained showed a total absence of faecal contamination indicators of both bacterial and viral origin, and also presented an excellent physico-chemical quality, independently of the quality of influent. Total aerobic bacteria counts revealed the problem of bacterial contamination in the membrane permeate zone, which could be controlled through daily chemical cleansing of the membrane. The chief problem presented by this type of system, applied as exclusive treatment, is low effectiveness in the retention of natural organic matter (NOM), in which respect the quality of the effluent was observed to depend on the quality of influent. This constitutes the principal limitation for applying the system to surface water due to the risk of disinfection by-products formation during the final post-chlorination. However, spiral wound ultrafiltration (SWUF) membranes could be used for low NOM reservoir water total treatment offering several advantages over conventional technologies.

New simulators for the optimum management and operation of wastewater treatment plant.

This paper presents the basic description and the first full-scale implementation of a new kind of simulator specially designed to facilitate and improve the management and operation of modern wastewater treatment plants (WWTP). This new kind of simulator for plant operation is specifically adapted to every WWTP and the software is developed considering the common needs of the operators in plant exploitation. The internal structure of the plant operation simulator is based on a complete connection between the real data and the mathematical model of the plant. The software is then able to perform the processing, storage and management of the plant data and to predict the evolution of the process reading the required inputs from its stored files. The results obtained with the first application recommend the implementation of this new kind of simulators for plant operation in other treatment plants. However, it is important to note that the application of this technology implies a systematic and rigorous methodology in the acquisition and processing of the most significant plant data.