Enhanced phosphorus solubility during anaerobic digestion

Abstract

Worldwide phosphorus (P) consumption has increased massively over the last century, to overcome soil nutrient constraints, and to feed an increasing population with an increasing level of urbanization. However, current P reserves are limited and are expected to deplete in 50-120 years. Hence, new techniques to source P from alternative resources are essential. Waste streams are the largest alternative resource of P, with much of the organic wastes containing P treated by anaerobic digestion (AD). However, P recovery from the digested waste streams is limited by in-reactor P precipitation, and inaccessibility of soil available P in sludge biosolids. In-reactor P precipitation increases the operational costs of a treatment facility due to scaling and precipitation, there are strong advantages in being able to recover P as a purified mineral product. In such circumstances, keeping high PO4 concentration during AD is vital to enhance its recovery post AD.This thesis aims to assess two different techniques to enhance PO4 concentration during AD from waste activated sludge (WAS): low pH and high pressure (enabling low pH). Low pH single stage AD was tested in both batch and continuous mode. In the batch study, biochemical methane potential tests were conducted for 51 days at a pH range of 5.0 to 7.2 in two separate sets (two different WAS samples collected from municipal WWTP). Low pH (< 5.7) caused a significant loss in the methane potential (B0) of up to 33%, with 3.6 times increase in PO4 concentration compared to the neutral pH (7 – 7.7), but with no major change in methane production rate coefficient (khyd). The loss in methaneyield was mainly due to decrease in hydrolytic capability rather than inhibition of methanogenesis with volatile fatty acids (VFAs) being < 300 mgCOD L-1 and soluble COD < 1300 mgCOD L-1 even at low pH. While pH did not influence the acetoclastic community (Methanosaeta dominated), it was the primary driver for the remaining community, and caused a loss of diversity and shift to Clostridia. To validate the results from batch conditions, continuous low pH AD was performed using similar substrate and pH conditions. The influence of the pH on PO4 concentration was similar in continuous and batch. It was found that the low pH (5.5) caused a significant increase in PO4 concentration up to 79% of the total P, while methane yield was reduced by 50%. VFAs and SCOD concentrations increased from 40 to 504 mg L-1 and 600 to 2017 mg L-1 respectively, as the pH was reduced from 7.0 to 5.5. Higher concentration of propionic acid (370 – 430 mg L-1 ) was recorded at low pH (< 5.5). The reduction in methane yield was associated with a shift in microbial community and decreased destruction of particulate organics. Acidogens dominated at low pH (< 6.0), while methanogens decreased by 88% at pH 5.5 compared to neutral pH. Apart from the loss in methanogenic and hydrolytic capacity, continuous acid dosing to maintain low pH condition was identified as a key limitation with this technology. To assess an alternative method to avoid acid dosing, operation under pressure was assessed (at 1, 2, 4 and 6 bar absolute pressure). The average PO4 concentration increased to 51.2 ± 0.01, 56.4 ± 0.05, 65.4 ± 0.1, and 75.3 ± 0.05% at 1 (control), 2, 4, and 6 bar respectively. The specific methane yield was 66.8 ± 3.6, 47.4 ± 4, and 58.5 ± 3.5 L-CH4 kg-VSfed-1 at 2, 4, and 6 bar respectively (averaging 40% increase compared to 1 bar), but VSD and COD removal was unaffected, indicating better gas capture. Total VFAs concentration were below 15 mg L-1 at all conditions. The CO2 content were 27.6, 19.8, 16.7 and 13.5% at 1 (control), 2, 4, and 6 bar respectively (with the balance being methane). Increased pressure caused a substantial change in Archaeal populations, to novel clades, without substantial change in function. Increased PO4 concentration at high pressure was due to the combined effect of low pH conditions and dissociation of PO4 based precipitants caused by increased ion activity. Overall, auto generative high-pressure AD is a chemical free technique to improve PO4 concentration and methane content in the biogas with the main barrier being increased capital cost. Low pH (up to 5.5) and high pressure (up to 6 bar) AD is recommendable to enhance P recovery, where low pH AD can be integrated without changing current infrastructure, while AD at a pressure up to 6 bar may require specialized reactor design.