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 methane yield 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...

Enhancing soluble phosphate concentration in sludge liquor by pressurised anaerobic digestion

“…The total VFA concentration at all pressure conditions and control was below 12 mg L -1 , without accumulation (refer supplementary material 3). The SCOD concentration at all pressure conditions was always lower than the control, contrasting with previous work at artificially induced low pH (Latif et al 2015(Latif et al , 2017. This indicates that the low pH and/or high-pressure conditions are reducing hydrolytic capability without inducing accumulation of VFA and other soluble organics.…”

“…There is limited literature available on enhancing soluble PO 4 in anaerobic digester effluents to enable its recovery. Options such as EDTA and ion exchange resins are uneconomic (Latif 2017).…”

A Novel Simple Dipping-Nebulizing Water Absorption for Biogas Purification

“…Water solvent at pH 6.78 enabled purification of biogas to reach a minimum percentage of CH4 concentration (> 65%), which is required to efficiently drive the load. This result is in agreement with the literature (Latif et al, 2014) that reports that CH4 production is maximum using a medium at pH 6.85. Consistently, the highest negative Δ for CO2 and H2S reduction was observed for biogas purification using water with pH 6.78.…”

“…The output of gas constituents with low solubility in water, including CH4 and O2, is also affected by pH. Thus, increasing pH results in an increase in the solubility of CH4 and O2 (Latif et al, 2014;Gross and Cravotta III, 2016). In this observation, at a higher pH (7.28), the concentration of CH4 and O2 in purified biogas was lower than that of the initial state of the biogas.…”

Ảnh hưởng của điều kiện pH đến quá trình phân hủy kỵ khí hai pha chất thải rắn hữu cơ có khả năng phân hủy sinh học

“…Ở các nghiên cứu có tải lượng lớn hơn, sự giao động của sản lượng và chất lượng khí biogas theo điều kiện pH là rất rõ ràng. Latif và cs [25]. đánh giá ảnh hưởng của pH đến bể phản ứng mê-tan trên nền cơ chất bùn hoạt tính.…”