F. Fdz-Polanco

Influence of pH over nitrifying biofilm activity in submerged biofilters

Abstract The influence of pH over nitrification in submerged biofilters has been studied through the observation of three pH effects over the nitrifying biofilm: activation-deactivation, substrate limitation, and free ammonia inhibition. Within a pH range of 5.0–9.0, a pH increase of one unit produce a 13% increase on the nitrification efficiency. A stoichiometry of 7.1 mg CaCO 3 /mg N was measured for the process, which became alkalinity limited below pH 5.0. The highest activity of ammonium oxidizers and the highest values of volatile attached solids (VAS) in the filter were obtained at pH 8.2. The concentration of volatile attached solids (VAS) was regulated by free ammonia inhibition, substrate limitation (NH 4 + concentration), and reactor hydrodynamics. Inhibition by free ammonia controlled bacteria activity at pH greater than 7.5, observing an increase of microorganisms concentration in the filter as a specific free ammonia concentration (mg NH 3free -N gVAS −1 ) decreased. A nitrite accumulation of up to 80–90% was obtained for specific inhibitory concentrations greater than 1.5 mg NH 3free -N gVAS −1 resulting from the selective inhibition of nitrite oxidizers.

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.

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.

Anaerobic treatment of cheese-production wastewater using a UASB reactor

Abstract Dairy wastewater comes mainly from the washing of the installations, and due to its medium content of organic matter it is well suited for a biological treatment, especially an anaerobic treatment. A laboratory scale (4 litres) UASB reactor was operated for more than a year, fed with wastewater from a cheese production industry. An organic loading rate (Bv) of 31 g COD/litre per day ( t = 1·7 h ) and a COD reduction near to 90% were reached, operating in steady-state conditions using a wastewater with a COD influent of 2050 mg/litre (BOD 1300 mg/litre). The effluent COD was 215 mg/litre (BOD 80 mg/litre). Organic loading rate peaks higher than 45 g COD/litre per day were sporadically reached with COD reductions of 70–80%. The reactor operated with great stability once it had developed a mature microbial population. Prior to this it was necessary to add some alkali in order to maintain the buffer capacity of the system. Biomass granulation occurred during the operation of the reactor after a sudden increase in organic loading rate. This allowed a more stable operation of the reactor.

New process for simultaneous removal of nitrogen and sulphur under anaerobic conditions

A granular activated carbon (GAC) anaerobic fluidised-bed reactor treating vinasse from an ethanol distillery of sugar beet molasses was operated for 90 days, the first 40 days of start-up followed by 50 days of operation at constant organic loading rate of 1.7g COD/Ld. The reactor showed good performance in terms of organic matter removal but an anomalous behaviour in terms of unusual high concentrations of molecular nitrogen in the biogas. The analysis of the different nitrogenous and sulphur compounds and the mass balances of these compounds in the liquid and gas phases clearly indicated an uncommon evolution of nitrogen and sulphur in the reactor. About 50% of the nitrogen entering the reactor as total Kjeldahl nitrogen (TKN) was removed from the liquid phase appearing as N2 in the gas phase. Simultaneously, only 20% of the S-SO4(2-) initially present in the influent appears as S-S2- in the effluent or S-H2S in the biogas, indicating that 80% of the sulphur is removed. This behaviour has not been reported previously in the literature. These observations may suggest a new anaerobic removal process of ammonia and sulphate according to an uncommon mechanism involving simultaneous anaerobic ammonium oxidation and sulphate reduction.

Effects of biofilm growth, gas and liquid velocities on the expansion of an anaerobic fluidized bed reactor (AFBR)

An anaerobic fluidized bed reactor, with 30 1 sepiolite particles as support material was used to study the influence of biofilm growth and biogas production over hydrodynamic behaviour. It was verified that although gas production transforms the AFBR from a two-phase fluidized bed to a three-phase gas-liquid-solid system, the formation of a gas phase has little effect over hydrodynamic behaviour, and therefore it is possible to describe the bed as like a two-phase solid-liquid system. However biofilm development has a more important effect on the hydrodynamic behaviour, so it is necessary to reduce the recirculation ratio to maintain a set expansion, when the attached volatile solids increase. The superficial velocity necessary to maintain a 20% bed expansion, decreased from 11.5 m/h for clean particles, to 9.0 m/h for 20 g/l attached volatile solids concentration. The growth of the biofilm varies throughout the reactor, increasing the natural variation in the size of the particles in the bed.

Low temperature treatment of municipal sewage in anaerobic fluidized bed reactors

Abstract The anaerobic fluidized bed reactor (AFBR) appears to be most promising for the treatment of low strength wastes, such as municipal sewage, at low temperature, since the process is able to maintain a large mass of active microorganisms and provides effective removal of TSS. The study is divided in three parts. The objective of the first part is to characterize the effect of decreasing temperature on the performance of two mature AFBR reactors. The second part presents the data from 220 days of operation at 10°C; and in the third part two start-ups, with and without inoculum at 15°C, are evaluated. A gradual temperature decrease from 20 to 5°C, allowing the microorganisms to acclimate to the new lower temperature, did not have a great effect on effluent quality. However a great accumulation of TSS was observed in the top of the fluidized bed. At 10°C, and a hydraulic retention time of 1.5 h, 70% of TCOD removal was achieved. It is possible to start-up the AFBR at 15°C without inoculation; however, at least 4 months is required to get good quality effluents.

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.

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.