Studies with an anaerobic expanded bed reactor comparing the performances achieved with synthetic waste and domestic sewage

The performance of an anaerobic expanded bed reactor has been examined during the treatment of a synthetic low strength waste whose consistency and composition were constant. The organic loading rates used were in the range 0.2–5 kg total organic carbon m^?3 day^?1 and the removal efficiencies varied from 89 to 52%. Comparing these results with those obtained previously for the treatment of domestic sewage, whose strength and composition was very variable, showed that not only can higher removal efficiencies be achieved with the synthetic waste but also that a significantly better (R² > 0.85, as opposed to <0.5) correlation between data could be obtained. This indicates the potential danger of using synthetic feedstocks for the prediction of reactor performance under real conditions.     

Developments in pre-treatment methods to improve anaerobic digestion of sewage sludge

During wastewater treatment, most organic matter is transferred to a solid phase commonly known as sludge or biosolids. The high cost of sludge management and the growing interest in alternative energy sources have prompted proposals for different strategies to optimize biogas production during anaerobic sludge treatment. Because of the high solid content and complex structure of sludge-derived organic matter, methane production during digestion is limited at the hydrolysis step. Therefore, large digester volume and long retention times of over 20 days are necessary to achieve adequate stabilization. Pre-treatments can be used to hydrolyze sludge and consequently improve biogas production, solids removal and sludge quality after digestion. This paper reviews the main pre-treatment processes, with emphasis on the most recent developments. An overview of the different technologies is presented, discussing their effects on sludge properties and anaerobic digestion. Future challenges and concerns related to pre-treatment assessment and implementation are also addressed.     

Treatment of slaughterhouse wastewater in a UASB reactor and an anaerobic filter

A study was performed to assess the feasibility of anaerobic treatment of slaughterhouse wastewaters in a UASB (Upflow Anaerobic Sludge Blanket) reactor and in an AF (Anaerobic Filter). Among the different streams generated, the slaughter line showed the highest organic content with an average COD of 8000 mg/l, of which 70% was proteins. The suspended solids content represented between 15 and 30% of the COD. Both reactors had a working volume of 21. They were operated at 37°C. The UASB reactor was run at OLR (Organic Loading Rates) of 1–6.5 kg COD/m³/day. The COD removal was 90% for OLR up to 5 kg COD/m³/day and 60% for an OLR of 6.5 kg COD/m³/day. For similar organic loading rates, the AF showed lower removal efficiencies and lower percentages of methanization. At higher OLR sludge, flotation occurred and consequently the active biomass was washed out from the filter. The results indicated that anaerobic treatment systems are applicable to slaughterhouse wastewaters and that the UASB reactor shows a better performance, giving higher COD removal efficiencies than the AF.     

Treatment of domestic wastewater in an up-flow anaerobic sludge blanket reactor followed by moving bed biofilm reactor

The performance of a laboratory-scale sewage treatment system composed of an up-flow anaerobic sludge blanket (UASB) reactor and a moving bed biofilm reactor (MBBR) at a temperature of (22–35 °C) was evaluated. The entire treatment system was operated at different hydraulic retention times (HRT’s) of 13.3, 10 and 5.0 h. An overall reduction of 80–86% for COD_total; 51–73% for COD_colloidal and 20–55% for COD_soluble was found at a total HRT of 5–10 h, respectively. By prolonging the HRT to 13.3 h, the removal efficiencies of COD_total, COD_colloidal and COD_soluble increased up to 92, 89 and 80%, respectively. However, the removal efficiency of COD_suspended in the combined system remained unaffected when increasing the total HRT from 5 to 10 h and from 10 to 13.3 h. This indicates that, the removal of COD_suspended was independent on the imposed HRT. Ammonia-nitrogen removal in MBBR treating UASB reactor effluent was significantly influenced by organic loading rate (OLR). 62% of ammonia was eliminated at OLR of 4.6 g COD m⁻² day⁻¹. The removal efficiency was decreased by a value of 34 and 43% at a higher OLR’s of 7.4 and 17.8 g COD m⁻² day⁻¹, respectively. The mean overall residual counts of faecal coliform in the final effluent were 8.9 × 10⁴ MPN per 100 ml at a HRT of 13.3 h, 4.9 × 10⁵ MPN per 100 ml at a HRT of 10 h and 9.4 × 10⁵ MPN per 100 ml at a HRT of 5.0 h, corresponding to overall log₁₀ reduction of 2.3, 1.4 and 0.7, respectively. The discharged sludge from UASB–MBBR exerts an excellent settling property. Moreover, the mean value of the net sludge yield was only 6% in UASB reactor and 7% in the MBBR of the total influent COD at a total HRT of 13.3 h. Accordingly, the use of the combined UASB–MBBR system for sewage treatment is recommended at a total HRT of 13.3 h.     

Layered structure of bacterial aggregates produced in an upflow anaerobic sludge bed and filter reactor

The ultrastructure of bacterial granules that were maintained in an upflow anaerobic sludge bed and filter reactor was examined. The reactor was fed a sucrose medium, and it was operated at 35 degrees C. Scanning and transmission electron microscopy revealed that the granular aggregates were three-layered structures. The exterior layer of the granule contained a very heterogeneous population that included rods, cocci, and filaments of various sizes. The middle layer consisted of a slightly less heterogeneous population than the exterior layer. A more ordered arrangement, made up predominantly of bacterial rods, was evident in this second layer. The third layer formed the internal core of the granules. It consisted of large numbers of Methanothrix-like cells. Large cavities, indicative of vigorous gas production, were evident in the third layer. On the basis of these ultrastructural results, a model that presents a possible explanation of granule development is offered.     

Starting-up an anaerobic hybrid filter for the fermentation of wastewater from food industry

The wastewater from a food processing factory, characterised by fluctuations of flow rate, organic strength, and pH, were originally treated by a traditional suspended-biomass digester working at about 25?°C. In order to improve the digester efficiency, either in terms of degradation ability or biogas production yield, a set of tests has been carried out on laboratory scale, whose results indicated the way to correctly transform it into an anaerobic hybrid filter. The unacceptable conversion yield of organic substances into biogas observed in the original system has been improved by the presence of the filling medium, due to a marked increase in biomass retention time. The start-up of anaerobic digestion has been studied in this reactor at two different temperatures (25 and 30?°C), in order to evaluate the possible advantage of heating the system, simulating continuous variations in feed strength, pH, and composition.     

Performance of a hybrid anaerobic reactor,combining a sludge blanket and a filter,treating slaughterhouse wastewater

A column reactor, in which the bottom two-thirds were occupied by a sludge blanket and the upper one-third by submerged clay rings, was evaluated using slaughterhouse wastewater as substrate. The reactor was operated at 35°C at loading rates varying from 5 g to 45 g chemical oxygen demand (COD) 1^–1 × day^–1 at an influent concentration of 2450 mg COD 1^–1. A maximum substrate removal rate of 32 g COD 1^–1 × day^–1, coupled with a methane production rate of 6.91 × 1^–1 × day^–1 (STP), was obtained. This removal rate is significantly higher than those previously reported. The rate of substrate utilization by the biomass was 1.22 g COD (g volatile suspended solids)^–1 day^–1. COD removal was over 96% with loading rates up to 25 g COD 1^–1 × day^–1, at higher loading rates performance decreased rapidly. It was found that the filter element of the reactor was highly efficient in retaining biomass, leading to a biomass accumulation yield coefficient of 0.029 g volatile suspended solids g^–1 COD, higher than reported previously for either upflow anaerobic sludge-blanket reactors or anaerobic filters operating independently.     

Treatment of sewage water from tourist areas by anaerobic fixed-bed reactor

A laboratory-scale anaerobic fixed-bed reactor, operating at ambient temperature (30 to 35°C), was used to treat sewage water from tourist areas in Cuba at hydraulic retention times (HRT) ranging from 4 to 72 h. The total chemical oxygen demand (T-COD), total biological oxygen demand (T-BOD) and total suspended solids removal varied between 30 and 80%, 40 and 95% and 25 and 80%, respectively. Total and faecal coliforms were reduced by 98.1 to 99.9% and by 99.0% to 99.9% respectively, despite the marked decrease in HRT from 72 to 4 h.     

Production of vitamin B12 in an upflow anaerobic filter continuous reactor using Acetobacterium sp.

The accumulation of biofilm by Acetobacterium sp. during continuous culture in an upflow anaerobic filter (UAF) growing on methanol-formate was the result of space velocity and inlet concentrations of substrate and Co⁺². To achieve good development of biofilm, a space velocity of 0.38 h^–1, inlet substrate concentrations of 125 mM of both methanol and formate, and Co⁺² at 0.16 mM were required. Cell productivities in the effluent of the UAF-reactor were about 6-fold higher than in chemostat cultures (0.20 g l^–1 h^–1 for UAF and 0.035 g l^–1 h^–1 for chemostat) (previous studies), and the maximum vitamin B₁₂ specific concentration was 5.1 mg g cell^–1.     

Layered structure of bacterial aggregates produced in an upflow anaerobic sludge bed and filter reactor.

The ultrastructure of bacterial granules that were maintained in an upflow anaerobic sludge bed and filter reactor was examined. The reactor was fed a sucrose medium, and it was operated at 35 degrees C. Scanning and transmission electron microscopy revealed that the granular aggregates were three-layered structures. The exterior layer of the granule contained a very heterogeneous population that included rods, cocci, and filaments of various sizes. The middle layer consisted of a slightly less heterogeneous population than the exterior layer. A more ordered arrangement, made up predominantly of bacterial rods, was evident in this second layer. The third layer formed the internal core of the granules. It consisted of large numbers of Methanothrix-like cells. Large cavities, indicative of vigorous gas production, were evident in the third layer. On the basis of these ultrastructural results, a model that presents a possible explanation of granule development is offered.     

Composting domestic sewage sludge with natural zeolites in a rotary drum reactor

This work aimed the influence of zeolites addition on a sludge-straw composting process using a pilot-scale rotary drum reactor. The type and concentration of three commercial natural zeolites were considered: a mordenite and two clinoptilolites (Klinolith and Zeocat). Mordenite caused the greatest carbon removal (58%), while the clinoptilolites halved losses of ammonium. All zeolites removed 100% of Ni, Cr, Pb, and significant amounts (more than 60%) of Cu, Zn and Hg. Zeocat displayed the greatest retention of ammonium and metals, and retention efficiencies increased as Zeocat concentration increased. The addition of 10% Zeocat produced compost compliant with Spanish regulations. Zeolites were separated from the final compost, and leaching studies suggested that zeolites leachates contained very low metals concentrations (<1 mg/kg). Thus, the final compost could be applied directly to soil, or metal-polluted zeolites could be separated from the compost prior to application. The different options have been discussed.     

Performance and biomass retention of an upflow anaerobic reactor combining a sludge blanket and a filter

Summary The performance and biomass retention of a 4.2 L new hybrid reactor (upflow blanket filter, UBF) at 27 °C were determined at loading rates of 5 to 51 g Chemical Oxygen Demand (C0D)/L.d with sugar wastes of 2500 mg C0D/L strength. Maximum removal rates of 34 g C0D/L.d and C0D removal efficiency of over 93% were reached. The packing was very efficient in retaining biomass.     

Assessment of macroenergetic parameters for an anaerobic upflow biomass bed and filter (UBF) reactor

This article reports the steady-state performance of two hybrid anaerobic digesters treating soluble synthetic sugar wastes of 1 and 0.5% strength and the assessment of the associated macroenergetic parameters (growth yield, so-called maintenance coefficient). A theoretical development shows a "nongrowth" parameter concept to be more appropriate than maintenance or decay. Combined energy and mass balances are used to develop a model for growth rate which compares well with experimental data. The COD removal efficiency had no significant effect on growth yield and the maintenance parameter, although a dual combined balance indicated the possibility of such an effect. Macroenergetic parameters did not vary significantly with the specific feeding rate of the system. We thus conclude that a single model may be used over a broad range of feeding and performance conditions.     

Performance of an upflow anaerobic reactor combining a sludge blanket and a filter treating sugar waste

A new hybrid reactor, the upflow blanket filter (UBF), which combined on open volume in the bottom two-thirds of the reactor for a sludge blanket and submerged plastic rings (Flexiring, Koch Inc., 235 m(2)/m(3)) in the upper one-third of the reactor volume, was studied. This UBF reactor was operated at 27 degrees C at loading rates varying from 5 to 51 g chemical oxygen demand (COD)/L d with soluble sugar wastewater (2500 mg COD/L). Maximum removal rates of 34 g COD/L d and CH(4) production rates of 7 vol/vol d [standard temperature and pressure (STP)] were obtained. The biomass activity was about 1.2 g COD/g volatile suspended solids per day. Conversion (based on effluent soluble COD) was over 93% with loading rates up to 26 g COD/L d. At higher loading rates conversion decreased rapidly. The packing was very efficient in retaining biomass.     

Growth characteristic of anaerobic ammonium-oxidizing bacteria in an anaerobic biological filtrated reactor

The doubling time of anaerobic ammonium-oxidizing (anammox) bacteria in an anaerobic biological filtrated (ABF) reactor was determined. Fluorescence in situ hybridization analysis was used to detect and count anammox bacteria cells in anammox sludge. As a result, the populations of anammox bacteria at 14th and 21st days were 1.1×10⁶ and 1.7×10⁷ cells/ml reactor, respectively. From these results, the doubling time of anammox bacteria was calculated as 1.8 days, and the specific growth rate (μ) was 0.39 day⁻¹. This result indicated that the anammox bacteria have higher growth rate than the reported value (doubling time, 11 days). Furthermore, it was clearly demonstrated that nitrogen conversion rate was proportional to the population of anammox bacteria. Maintaining the ideal environment for the growth of anammox bacteria in the ABF reactor might lead to faster growth. This is the first report of the growth rate of anammox bacteria based on the direct counting of anammox bacteria.     

Treatment of low strength industrial cluster wastewater by anaerobic hybrid reactor

The study was aimed at treating the complex, combined wastewater generated in Mangolpuri industrial cluster. It was considered as a low strength wastewater with respect to its organic content. Anaerobic treatment of this wastewater was studied using an anaerobic hybrid reactor (AHR) which combined the best features of both the upflow anaerobic sludge blanket (UASB) reactor and anaerobic fluidized bed rector (AFBR). The performance of the reactor under different organic and hydraulic loading rates were studied. The COD removal reached 94% at an organic loading rate (OLR) of 2.08 kg COD m(-3)d(-1) at an hydraulic retention time (HRT) of 6.0 h. The granules developed were characterized in terms of their diameter and terminal settling velocity.     

Investigation of the FeFe-hydrogenase gene diversity combined with phylogenetic microbial community analysis of an anaerobic domestic sewage sludge

Biological hydrogen production through the anaerobic digestion is an environmental friendly alternative for satisfying future hydrogen demands. Microorganisms residing into waste water treatment plants are far from being exhaustively characterized and surveys on hydrogen production through FeFe-hydrogenase in such ecosystems are scarce. This study combined the analysis of 16S rRNA and [FeFe]-hydrogenase (hydA) genes with statistical tools to estimate richness and diversity of the microbial community of a domestic sewage treatment plant at the phylogenetic and functional levels. Archaeal groups were represented by 69 % of sequences assigned to Methanosarcinales and the remaining belonged to Methanomicrobiales. Within the bacterial library, 136 operational taxonomic units (OTUs) were distributed into 9 phyla, being 86 OTUs related to uncultivated bacteria. From these, 25 OTUs represented potential novel taxa within Synergistetes. Proteobacteria was the most predominant (36 % of the OTUs) and diversified phylogenetic group in the bacterial library, most of them assigned to the class Betaproteobacteria. Twenty-two putative hydA sequences were recovered into four distinct clusters and most of them were more closely related to each other than with sequences retrieved from databases, indicating they are hitherto undetected [Fe–Fe]-hydrogenase gene sequences. The richness estimates revealed that the number of sampled sequences was enough for full coverage of the archaeal diversity but not sufficient to cover both bacterial and hydA gene diversities. The results confirmed a great richness and diversity of bacterial and hydA sequences retrieved from the sewage sludge sample, suggesting such environment as a potential reservoir of new hydrogenase genes for biotechnological exploration.     

Transformation of tetrachloroethene in an upflow anaerobic sludgeblanket reactor

  Reductive dechlorination of tetrachloroethene was studied in a mesophilic upflow anaerobic sludge blanket reactor. Operating the reactor in batch mode the dynamic transformation of tetrachloroethene, trichloroethene and dichloroethene (DCE) was monitored. Tetrachloroethene was reductively dechlorinated to trichloroethene, which again was dechlorinated at the same rate as DCE was produced. DCE showed a lag period of 40 h before transformation was observed. During normal reactor operation trans-1,2-DCE was the major DCE isomer, followed by cis-1,2-DCE. Small amounts of 1,1-DCE but no vinyl chloride were detected. When the influent tetrachloroethene concentration was increased from 4.6 μM to 27 μM, the transformation rate increased, indicating that the system was not saturated with tetrachloroethene. The main organic component in the effluent was acetate, indicating that the aceticlastic methane-producing bacteria were inhibited by the chlorinated ethenes. Received: 29 July 1996 / Received revision: 13 September 1996 / Accepted: 13 September 1996     

Performance and characteristics of an anaerobic baffled reactor

The performance and the characteristics of a laboratory scale anaerobic baffled reactor (ABR) were investigated using synthetic wastewater. The experimental results showed that among different volatile fatty acids (VFAs), acetate was the main intermediate of acidogenic degradation of glucose. The VFA concentration decreased longitudinally down the reactor. The analysis of the biogas composition revealed that methane concentration increased steadily from compartment 1 to 5, while hydrogen content decreased in the first compartments. There was no detectable hydrogen in the last two compartments. The methane-producing activity of anaerobic sludge in different compartments depended on the substrate, which suggests that the proper anaerobic consortium in each separate compartment was developed according to the substrate(s) availability and the specific environmental conditions. The ABR has the potential to provide a higher efficiency at higher loading rates and be applicable for extreme environmental conditions and inhibitory compounds.     

Feasibility study of a pilot-scale sewage treatment system combining an up-flow anaerobic sludge blanket (UASB) and an aerated fixed bed (AFB) reactor at ambient temperature

A feasibility test of a 17 m3-pilot-scale sewage treatment system was carried out by continuous feeding of raw municipal sewage under ambient temperature conditions. The system consisted of a UASB and an aerated fixed bed reactor. Some of the effluent from the fixed bed reactor was returned to the UASB influent in order to provide a sulfate source. The total BOD of 148-162 mg l(-1) in the influent was reduced to a more desirable 11-25 mg l(-1) in the final effluent. The levels of methane-producing activity from acetate and H2/CO2 gas at 10 degrees C were only 2% and 0% of those at 35 degrees C, respectively. On the other hand, the sulfate-reducing activity levels of the UASB sludge were relatively high at 10 degrees C, for example, 18% for acetate and 9% for H2/CO2 gas, compared to the activity levels at 35 degrees C. Therefore, BOD oxidization by sulfate reduction in the UASB was greater than that by methane production under low temperature conditions. This sulfate-reducing activity tended to be proportional to the copy number of adenosine-5'-phosphosulfate (APS) reductase genes in DNA extracted from the sludge.