Patrick Reynolds

PhD Thesis, "Support Matrix and Feed Flow Effects in Anaerobic Fixed-Bed Reactors", completed under the supervision of Professor Emer Colleran at the University College Galway, Ireland in 1987. This involved examined the relationship between the potential for methane production in anaerobic digesters operated using agricultural waste in the following ways:

• Upflow - Fixed Film Packed Reactor
• Upflow Hybrid/Sludge-Bed Fixed Film - Effect of varying the matrix quantity
• Downlflow Fixed Film.

Anaerobic reactors were studied for optimisation in:

• Start-up
• Long term Operation
• Methanogenic Activity in attached and non-attached sludges
• Liquid flow dispersion

Together with Professor Colleran, developed and improved upon specific methanogenic activity tests that existed during the 1980s. These included the Cofactor F₄₂₀ assay and the measurement of specific methanogenic potential of sludges.

During the 1990s collaborated with Professor David Nedwell at the University of Essex, UK in the development of anaerobic treatment of landfill leachates under both methanogenic and sulphate reducing conditions. The latter novel form of anaerobic digestion has been further studied by researchers. Methanogenic bioreactors treating organic wastewater containing sulfate can be negatively affected by the sulfide produced. However, it is possible to combine methanogenesis and sulfate reduction when adequate measures are applied. Heavy metals in effluents or in waste streams (such as Cd, Cu, Cd, Fe, Ni, Pb, and Zn) can be removed by precipitation with sulfide. metals can be selectively precipitated due to their differing solubility products, i.e. the metals they can be selectively precipitated/recovered in order to be reused.

Treatment of Landfill Leachate by Methanogenic and Sulphate-based Anaerobic Digestion

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High concentrations of sulphate in wastewater streams subjected to anaerobic digestion have been considered to be undesirable because of the production of sulphide during digestion, which can inhibit methanogenesis. Appreciable sulphate may be present normally in many industrial effluents, and anaerobic digestion of such effleuents results in low biogas formation and high concentrations of sulphide in the effluent.

Sulphate-reducing bacteria (SRB) compete with methanogenic bacteria (MB) for substrates such as acetate and hydrogen, outcompeting MB when sulphate is present in the environment. While the methanogenic bacteria metabolize only a restricted range of substrates (principally acetate and hydrogen), the SRB metabolize a far wider range of substrates, including long chain fatty acids, aromatic compounds etc. The SRB might be expected, therefore, to be more resilient to challenge by novel organic molecules than the MB, and digestion based upon sulphate-reduction therefore more resilient than methanogenic digestion. Furthermore, the production of sulphide during digestion may precipitate many metals as their insoluble sulfides, a process which has been used to reduce high metal content in waste waters. In contrast, methanogenic digestion has a reputation for being sensitive to toxicity by organic inhibitors such as toluene or chloroform and by metals. Inhibition of methanogenesis can also be caused by organic overload when the rates of acid production exceeds methane production, leading to decreased pH and souring of the digester.

This study was designed to investigate the effect of high sulphate concentrations, which may be present in landfill leachates. The study was commissioned by the UK Department of Environment (now Defra) who needed to understand the impact of elevated sulphate levels in landfill leachate, which would be expected to occur if coal containing high sulphate levels was imported from outside of the UK, and the disposal option for the processed coal waste (to remove sulphate) was co-disposal with household and/or industrial waste in landfill. This work investigated the treatability of landfill leachate with elevated sulphate and heavy metals by anaerobic digestion, comparing methanogenic with sulphate-based digestion, in upflow anaerobic digesters where biomass was retained as biofilms on support material.

RESULTS

• Steady-state removal efficiencies of COD under methanogenic digestion was 81-97%, depending upon organic loading rate, and effective treatment occurred up to a volumetric COD loading rate of 3.75 kg COD.m^-3.d^-1
• In sulphate-reducing digesters SO₄ was converted stoichiometrically to dissolved H₂S, driving organic matter mineralization. Sulphate concentrations of 35, 105 and 175 mM in the feed resulted in SO₄ removal efficiencies of 92, 52 and 35%, respectively.
• Steady state removal efficiencies of organic COD in the sulphate-reducing digesters averaged 62% at 35 mM, 80% at 105 mM, and 84% at 175 mM feed SO4 concentrations.
• The ratio of COD:SO4 (g/g) utilized was dependent upon the concentration of SO4 present, and at 35 mM SO₄ was 0.67. At an organic loading rate <1 kg COD.m^-3.d^-1 the sulphate-reducing digesters offered organic removal efflciencies comparable to methanogenic digesters, but were less effective at higher loading rates.
• Propionate accounted for the majority of the residual COD in the effluents from the methanogenic digesters, but acetate in the effluent from the sulphate-reducing digesters.
• Removal of sulphide from the reactors was achieved by addition of iron and by gassing off sulphide, but there was no increase in COD removal after sulphide removal which indicated that sulphide toxicity was not significant.
• Both methanogenic and sulphate-reducing reactors completely removed up to 100 ppm Cu, Mn, Ni and Zn added in the feed, indicating that even methanogenic reactors produce sufficient sulphide to precipitate these metals completely.
• The two types of digesters used in this study each had their own advantages and disadvantages. The methanogenic digesters under stress loading accumulated propionate, while the sulphate digesters accumulated acetate (Table below). When combined together as a two-stage process the final effluent fell to a very low COD as the initial sulphate-reducing stage effectively converted C3-C8 fatty acids to acetate, and the relatively high residual acetate from the first stage was removed by the second methanogenic stage. Overall, 97% removal of organic COD was removed. Thus it appears that a two stage process might harness the advantages of both types of reactor to produce a final effluent with an extremely low COD.

Methanogenic Activity Measurements

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Development of a Rapid Methoanogenic Activity Test Procedure
The specific methanogenic activity of anaerobic sludges is usually assayed by gas chromatographic analysis for methane in the headspace of sealed test vials. Gas is routinely sampled with a pressure lock syringe, which allows quantification independent of the pressure prevailing in the vials. An alternative method was developed using pressure transducer to monitor the increase in headspace gas pressure as CH₄/CO₂. Application of a simple formula, related the increase at each sample point to millilitres of CH₄ produced, gave values for the specific methanogenic activity of a variety of anaerobic sludges, which were in good agreement with the values obtained by the more labor-intensive gas chromatographic method. This method has been subsequently developed by other researchers to measure hydrogenotrophic activity of sludges and for the methanogenic activity of pure and mixed cultures of methanogenic bacteria. The diagram on the left was borrowed from the PhD Thesis of Maria de Sousa Pereira (2003), which details the methodology and it's further developments since its first design and use by Dr Reynolds in 1986.

Coenzyme F₄₂₀ Content of Methanogenic Sludges
The levels of this coenzyme have been used in anaerobic digestion studies as a measure of the total methanogen content of digester or seed sludges and as a predictive value of their potential methanogenic activity. Determination of the coenzyme F₄₂₀ content of anaerobic sludges from operational digesters by the various methods reported in the literature was shown to give conflicting results. The main methods in general use were compared and individual steps in the extraction and quantitation protocols were evaluated, modified and optimized. A standardized procedure, which gave accurate and reproducible results for all sludge samples tested, was developed.

Leachate Treatment Field Trials

This study, which was carried out in 1994, was intended to demonstrate the operational features of a pilot-scale anaerobic digester linked to a secondary stage nitrification plant for the on-site treatment of landfill leachate.

During the first year of operation, excessive inorganic scale accumulation on the interior of a 4m³ Upflow Anaerobic Sludge-Bed/Filter reactor and pipework surfaces resulted in operational problems, and forced the eventual shutdown of the pilot-plant. These studies highlighted the requirement for iron removal prior to anaerobic treatment, as evidenced by the low percentage of volatile matter (17.5%) and the high proportion of iron (10.5% w/w) in the sludge after operation for one year. Pre-treatment of the metals within the leachate was initiated by the addition of alkali, for metal precipitation, followed by neutralisation with acid. The maximum efficiency of the pre-treatment system was between 0.1 and 0.15 kgFe.m^-3.d^-1, which was approximately 60% of the maximum loading rate. Pre-treatment with alkali also removed calcium, zinc and phosphate to varying amounts. Results presented in this study suggested that a COD:phosphate ratio of 1000:1 was sufficient to support anaerobic decomposition within the digester, under the conditions used.

Extended operation of the pilot-scale digester at a HRT of 2 days resulted in a maximum treatment of 4 kgCOD.m^-3.d^-1, equating to 55% removal of COD. The relatively low organic treatment efficiency observed in comparison to previous laboratory studies, where treatment efficiencies in excess of 10 kgCOD.m^-3.d^-1 have been recorded, was shown to be caused by: (i) the variable leachate strength; (ii) heavy metal concentration leading to blockages; (iii) leachate feed arrangement to the reactor; and, (iv) poor hydraulic contact with the sludge.

The results of this study showed that the process of nitrification was inhibited at COD and BOD loadings above 0.5 and 0.25 kg.m^-3.d^-1, respectively. The study proved the necessity for a secondary stage aerobic treatment to metabolise residual organic material from the digester effluent. Treatment of residual organics in the digester effluent by the activated sludge unit provided an effluent of nutrient quality sufficient for nitrification. Subsequent studies showed that operation of the anaerobic digester, activated sludge unit and a Rotating Bological Contactor (RBC) in series proved successful as a treatment process for the removal of organics and the nitrification of ammonia from leachate.

This study further demonstrated the possibility of treating high concentrations of nitrate, resulting from the nitrification of leachate. The use of an external carbon source for denitrification was unnecessary. In terms of organic and nitrogen removal, high efficiencies were achieved. A four stage, organic-nitrification-denitrification system used in this study was capable of reducing COD, BOD and ammoniacal-nitrogen concentrations of 15,000, 9,240 and 1,100 mg.l^-1 to 1,300, 35 and 45 mg.l^-1, respectively.

The information supplied drew attention to those process features of the trial which have positively and negitavely influenced the process performance. The use of an anaerobic upflow sludge bed/filter and an attached-growth system for denitrification, such as a fluidised-bed reactor, were proposed as improvements to system used in this study.