Fate of Trace Metals in Anaerobic Digestion.

A challenging, and largely uncharted, area of research in the field of anaerobic digestion science and technology is in understanding the roles of trace metals in enabling biogas production. This is a major knowledge gap and a multifaceted problem involving metal chemistry; physical interactions of metal and solids; microbiology; and technology optimization. Moreover, the fate of trace metals, and the chemical speciation and transport of trace metals in environments--often agricultural lands receiving discharge waters from anaerobic digestion processes--simultaneously represents challenges for environmental protection and opportunities to close process loops in anaerobic digestion.

Velocity gradient as a tool to characterise the link between mixing and biogas production in anaerobic waste digesters.

Whilst the importance of mixing in anaerobic digesters to enhance process performance and gas production is well recognised, the specific effects of mixing regime on biogas production are not clear. Here, the velocity gradient is used to demonstrate the importance of minimally mixed zones in a digester, with computational fluid dynamics (CFD) models indicating that 20-85% of a laboratory-scale digester experiences local velocity gradients of less than 10 s⁻¹, dependent on mixing speed. Experimental results indicate that there is a threshold above which increased mixing speed (and hence velocity gradient) becomes counter-productive and biogas production falls. The effects of minimal mixing on digester microbiology are considered with the creation or destruction of localised pockets of high acetate concentration providing a possible explanation for the velocity gradient threshold. The identification of this threshold represents a valuable contribution to the understanding of the effects of mixing on gas production in anaerobic digesters.

Addition of Al and Fe salts during treatment of paper mill effluents to improve activated sludge settlement characteristics.

Metal salts, ferrous sulphate and aluminium chloride, were added to laboratory-scale activated sludge plant treating paper mill effluents to investigate the effect on settlement characteristics. Before treatment the sludge was filamentous, had stirred sludge volume index (SSVI) values in excess of 300 and was moderately hydrophobic. The use of FeSO4.7H2O took three weeks to reduce the SSVI to 90. Microscopic examination showed that Fe had converted the filamentous flocs into a compact structure. When the iron dosing was stopped, the sludge returned to its bulking state within four weeks. In a subsequent trial, the addition of AlCl3 initially resulted in an improvement of the settlement index but then caused deterioration of the sludge properties. It is possible that aluminium was overdosed and caused charge reversal, increasing the SSVI.

An examination of the treatment of iron-dosed waste activated sludge by anaerobic digestion.

Anaerobic digestion is an important sludge treatment process enabling stabilisation of the organic fraction of sewage sludge prior to land application. Any practice which might retard the anaerobic digestion process will jeopardize the stability of the resulting digested sludge. This paper reports on an investigation into the relative digestibility of iron-dosed waste activated sludge (WAS) from a sewage treatment works (STW) with chemical phosphorus removal (CPR), in comparison to WAS from a works without phosphorus removal. Two laboratory scale anaerobic digesters (51) were fed initially with non iron-dosed WAS (Works M) at a solids retention time of 19 days. After 2 months the iron-dosed CPR sludge (Works R) was introduced into the second digester, resulting in a 32% decrease in biogas production and an increase in the methane content of the biogas from an average of 74% to 81%. Pre-treatment of the CPR sludge with sodium sulphide and shear, both alone and in combination, caused the gas production to deteriorate further. Pre-acidification and pre-treatment with EDTA did result in an enhanced gas production but it was still not comparable with that of the digester being fed with non-iron-dosed sludge. The daily gas production was found to be linearly related to the amount of bound iron in the sludge.

Measuring metal and phosphorus speciation in P-rich anaerobic digesters.

High concentrations of soluble orthophosphate, magnesium and potassium are released during anaerobic digestion of biological phosphorus removal (BPR) sludge. This research was undertaken to investigate the effects of phosphorus enrichment on digester performance, metal and phosphorus speciation. High concentrations of soluble PO4-P (> 250 mg/l) were found to have a retarding effect on anaerobic digestion, reducing the rate of volatile solids digestion and methane production in comparison to control digesters. This was found to be reversible after a period of time, which was related to the amount of PO4-P added to the digesters, higher concentrations of PO4-P requiring more time for digester recovery. Addition of magnesium and potassium to the digesters, together with PO4-P, reduced the inhibitory effect of phosphorus enrichment but these digesters still showed lower rates of volatile solids digestion and methane production in comparison to the control digesters. Phosphorus enrichment resulted in extensive precipitation of calcium, magnesium and manganese, markedly reducing the soluble and easily available fractions of these metals. Other trace metals such as copper, zinc, chromium, nickel and cobalt actually showed increased levels of solubility as a result of phosphorus enrichment. This was thought to be caused by high levels of soluble organic carbon in the phosphorus-rich anaerobic digesters, which acted as organic ligands for metal complexation.