Technical and economic analysis of real anaerobic digester centrate by means of partial nitrification and sustainable heterotrophic denitrification.

The reliability of partial nitrification coupled with heterotrophic denitrification for the treatment of real anaerobic digester centrate produced in a wastewater treatment plant was technically and economically assessed in two sequencing batch reactors. Removal efficiencies above 90% were consistently achieved at N-ammonium loads above 1.2 g N L⁻¹ d⁻¹. Ethanol, affluent from a waste water treatment plant (biological treatment inlet) and a zero-cost liquid residue from a chemical industry containing polyethylene glycol and sorbitol were employed as carbon source for denitrification. In this last case, a total organic carbon (TOC) requirement of 4.5 g TOC g⁻¹ NO2⁻-N was calculated. The denitrification rate was 0.26 g NO2⁻-N g VSS⁻¹ d⁻¹ (VSS: volatile suspended solids). These results show that a carbon-rich waste can serve as a no-cost feed for denitrifying bioreactors. An in-depth economic analysis considering the main investment and operating costs of the process was developed, showing that it can suppose yearly savings above 50% with respect to the most widely used alternative of returning anaerobic digester centrate untreated to the head of the facility.

Startup and long-term performance of biotrickling filters packed with polyurethane foam and poplar wood chips treating a mixture of ethylmercaptan, H2S, and NH3.

UNLABELLED: Treatment of a mixture of NH3, H2S, and ethylmercaptan (EM) was investigated for more than 15 months in two biotrickling filters packed with poplar wood chips and polyurethane foam. Inlet loads ranging from 5 to 10 g N-NH3 m-3 hr-1, from 5 to 16 g S-H2S m-3 hr-1, and from 0 to 5 g EM m-3 hr-1 were applied. During startup, the biotrickling filter packed with polyurethane foam was re-inoculated due to reduced biomass retention as well as a stronger effect of nitrogen compounds inhibition compared with the biotrickling filter packed with poplar wood. Accurate pH control between 7 and 7.5 favored pollutants abatement. In the long run, complete NH3 removal in the gas phase was achieved in both reactors, while H2S removal efficiencies exceeded 90%. EM abatement was significantly different in both reactors. A systematically lower elimination capacity was found in the polyurethane foam bioreactor. N fractions in the liquid phase proved that high nitrification rates were reached throughout steady-state operation in both bioreactors. CO2 production showed the extent of the organic packing material degradation, which allowed estimating its service lifetime in around 2 years. In the long run, the bioreactor packed with the organic packing material had a lower stability. However, an economic analysis indicated that poplar wood chips are a competitive alternative to inorganic packing materials in biotrickling filters. IMPLICATIONS: We provide new insights in the use of organic packing materials in biotrickling filters for the treatment of H2S, NH3, and mercaptans and compare them with polyurethane foam, a packing commonly used in biotrickling filters. We found interesting features related with the startup of the reactors and parameterized both the performance under steady-state conditions and the influence of the gas contact time. We provide relevant conclusions in the profitability of organic packing materials under a biotrickling filter configuration, which is infrequent but proven reliable from our research results. The report is useful to designers and users of this technology.

Economical assessment of the design, construction and operation of open-bed biofilters for waste gas treatment.

A protocol was developed with the purpose of assessing the main costs implied in the set-up, operation and maintenance of a waste gas-treating conventional biofilter. The main operating parameters considered in the protocol were the empty bed residence time and the gas flow rate. A wide variety of investment and operating costs were considered. In order to check its reliability, the protocol was applied to a number of scenarios, with biofilter volumes ranging from 8.3 to 4000 m(3). Results show that total annualized costs were between 20,000 and 220,000 euro/year and directly dependent, among other factors, on the size of the system. Total investment and operating costs for average-size compost biofilters were around 60,000 euro and 20,000 euro/year, respectively, which are concordant with actual costs. Also, a sensitivity analysis was performed in order to assess the relative influence of a series of selected costs. Results prove that operating costs are those that influence the total annual costs to a higher extent. Also, packing material replacement costs contribute significantly to the total yearly costs in biofilters with a volume higher than 800 m(3). Among operating costs, the electricity consumption is the main influencing factor in biofilters with a gas flow rate above 50,000 m(3)/h, while labor costs are critical at lower gas flow rates. In addition, the use of a variety of packing materials commonly employed in biofiltration was assessed. According to the results obtained, special attention should be paid to the packing material selected, as it is the main parameter influencing the medium replacement costs, and one of the main factors affecting investment costs.

Removal of formaldehyde, methanol, dimethylether and carbon monoxide from waste gases of synthetic resin-producing industries.

The removal of mixtures of gas-phase pollutants released from formaldehyde- and formaldehyde resin-producing industries was studied in different bioreactor systems. The waste gases contained formaldehyde, methanol, dimethylether and carbon monoxide. The use of a hybrid two-stage bioreactor, composed of a biotrickling filter and a conventional biofilter connected in series, led to very high elimination capacities and removal efficiencies close to 100% for overall pollutant loads exceeding 600g m(-3)h(-1). The presence of low concentrations of dimethylether in the gaseous mixture did not have a significant effect on the removal of formaldehyde or methanol under our operating conditions, although moderate concentrations of these compounds did negatively affect the biodegradation of dimethylether. When a mixture of all four compounds, at concentrations around 100, 100, 50 and 50mg m(-3) for formaldehyde, methanol, carbon monoxide and dimethylether, respectively, was fed to a conventional biofilter, removal efficiencies higher than 80% were obtained for the first three pollutants at empty bed retention time values above 30s. On the other hand, dimethylether was removed to a lower extent, although its reduced environmental impact allows to conclude that these results were satisfactory.

Effect of key parameters on the removal of formaldehyde and methanol in gas-phase biotrickling filters.

The effect of some important operation parameters, as pH, pollutant load and composition of the nutrient media, on the biodegradation of a mixture of formaldehyde and methanol in a gas-phase biotrickling filter was studied. pH proved to affect the degradation of both compounds at moderately acidic values. Replacing ammonium with nitrate as nitrogen source in the liquid solution led to a slight decrease in performance, though this difference was not really significant. A slight decrease in the elimination rate was also observed when reducing the N-NO(3)(-) concentration to 60% of its original value. No interactions between the two pollutants were found under our working conditions.

Hydrodynamic behaviour and comparison of technologies for the removal of excess biomass in gas-phase biofilters.

The hydrodynamic behaviour of a biofilter fed toluene and packed with an inert carrier was evaluated on start-up and after long-term operation, using both methane and styrene as tracers in Residence Time Distribution experiments. Results indicated some deviation from ideal plug flow behaviour after 2-year operation. It was also observed that the retention time of VOCs gradually increased with time and was significantly longer than the average residence time of the bulk gas phase. Non-ideal hydrodynamic behaviour in packed beds may be due to excess biomass accumulation and affects both reactor modeling and performance. Therefore, several methods were studied for the removal of biomass after long-term biofilter operation: filling with water and draining, backwashing, and air sparging. Several flow rates and temperatures (20-60 degrees C) were applied using either water or different chemicals (NaOH, NaOCl, HTAB) in aqueous solution. Usually, higher flow rates and higher temperatures allowed the removal of more biomass, but the efficiency of biomass removal was highly dependent on the pressure drop reached before the treatment. The filling/draining method was the least efficient for biomass removal, although the treatment did basically not generate any biological inhibition. The efficiency of backwashing and air sparging was relatively similar and was more effective when adding chemicals. However, treatments with chemicals resulted in a significant decrease of the biofilter's performance immediately after applying the treatment, needing periods of several days to recover the original performance. The effect of manually mixing the packing material was also evaluated in duplicate experiments. Quite large amounts of biomass were removed but disruption of the filter bed was observed. Batch assays were performed simultaneously in order to support and quantify the observed inhibitory effects of the different chemicals and temperatures used during the treatments.

Optimization of nutrient supply in a downflow gas-phase biofilter packed with an inert carrier.

Several methodologies were tested to supply nutrients to a downflow biofilter packed with perlite and used to treat toluene-polluted air. Despite the presence of an inorganic carrier, elimination capacities of up to around 60 g/m(3) per hour could be maintained when a basal medium, containing nitrogen, phosphorus and potassium, was supplied once every fortnight or even once a month rather than once a week. Experimental results also indicated that the addition of vitamins or trace minerals to the basal aqueous medium hardly improved biofilter performance. Furthermore, the nutrient supply could be combined with a biomass control strategy, using air sparging, without any adverse effect on biofilter performance compared to supplying nutrients alone, and limiting the accumulation of excess biomass on the packing material. The performance of the biofilter was not significantly affected by temperature fluctuations between 25 and 33 degrees C.

Start-up and long-term performance of a gas phase biofilter packed with an inert carrier.

Several parameters affecting the performance and characteristics of alkylbenzene fed biofilters were checked when using perlite as inert carrier material. The influence of the inoculum on startup and long term performance was studied using both non defined as well as defined cultures. The inoculated pure cultures, which were still detected in the biofilter after several months operation, allowed shortening the start-up period Qualitative and quantitative biomass distribution appeared to be uneven along the biofilter bed in long term operation studies. The biofilter could withstand a drying period of several weeks although such operating condition led to a shift in dominant microbial populations in the biofilm which was followed by an increase of the biofilter performance. Although a water content of 55-60% appeared to be optimal with perlite, peformance data were still satisfactory down to a water content around 35-40%.