Effect of bioaugmentation on digestate metal concentrations in anaerobic digestion of sewage sludge

Heavy metal(loid)s and nutrients in sewage sludge in Portugal - Suitability for use in agricultural soils and assessment of potential risks

The presence of heavy metal(loid)s in sewage sludge is a cause of concern and an obstacle to its agricultural valorisation. This study analysed the elemental composition of sewage sludge from 42 Portuguese wastewater treatment plants (WWTPs) during summer and winter, investigating heavy metal(loid) contamination, nutrient content, and potential risks related to sludge application to agricultural soils. Levels of 8 heavy metal(loid)s were investigated, ranging from not detected (Hg) to 5120 mg kg-1 dw (Zn), decreasing in the order Zn > Cu > Cr > Ni > Pb > As>Cd > Hg. The legal requirements for agricultural use of sludge were overall met, but elevated levels of Zn and Cu, linked to industrial sources, exceeded the permitted limits in 3 WWTPs. On average, N, P, K, Mg, and Ca comprised 80 % of the sludge nutrient profile. No seasonal variations were found, but sludge composition varied with WWTP size, wastewater origin, and between thickened and digested samples. Environmental hazard indicators showed significant sludge contamination with Zn, Cu, and Cd. However, the geoaccumulation index, potential ecological risk indicators, and risk characterization ratios showed no significant risks to sludge-amended soils, assuming a single application of 5 tons ha-1. Human health risk assessment for workers handling sewage sludge identified dermal contact as the main route of exposure, with non-carcinogenic risk for Cr and carcinogenic risk for Ni and Cr at the highest reported levels. Sewage sludge produced in Portugal was considered suitable for agricultural use, provided that it is closely monitored and well-managed to meet the needs of crops and receiving soils, while mitigating environmental risks.

Characteristics, limitations and global regulations in the use of biogas digestate as fertilizer: A comprehensive overview

The utilization of biogas digestate, the effluent of anaerobic digestion (AD), as an organic fertilizer offers promising advances for sustainable agriculture, but it also presents critical challenges that require careful regulatory oversight. This review explores the wide characteristics range of digestate, key limitations, and regulatory frameworks shaping the use of biogas digestate as fertilizer. While digestate is a rich source of essential macro and micronutrients required for promoting plants growth, its application risks leading to nutrient overload, contamination from heavy metals, pathogens, antibiotics, microplastics, and emerging contaminants. By exploring the current regulations managing the utilization of biogas digestate as fertilizer, the EU limits digestate application to 170 kg N/ha/year, with a higher allowance in the UK (up to 250 kg N/ha/year). In other major biogas-producing countries, there is no specific limit for digestate application, as it varies depending on individual cases. Heavy metals and pathogens are satisfactorily regulated in the policies of these countries. However, no specific limits exist for antibiotics and microplastics, despite their significant impact on human health and the environment. Moreover, regulations concerning other potential chemicals are limited. Expanding these regulations is recommended to mitigate associated health and environmental risks.

Optimization of the Biomethane Production Process by Anaerobic Digestion of Wheat Straw Using Chemical Pretreatments Coupled with Ultrasonic Disintegration

Biomass is an attractive energy source that can be used for production of heat, power, and transport fuels and when produced and used on a sustainable basis, can make a large contribution to reducing greenhouse gas emissions. Anaerobic digestion (AD) is a suitable technology for reducing organic matter and generating bioenergy in the form of biogas. This study investigated the factors allowing the optimization of the process of biogas production from the digestion of wheat straw (WS). The statistical analysis of the experiments carried out showed that ultrasonic processing plays a fundamental role with the sonication density and solids concentration leading to improved characteristics of WS, reducing particle size, and increasing concentration of soluble chemical oxygen demand. The higher the sonicating power used, the more the waste particles are disrupted. The optimality obtained under mesophilic conditions for WS pretreated with 4% w/w (weight by weight) H2O2 at temperature 36 °C under 10 min of ultrasonication at 24 kHz with a power of 200 W improves the methane yield by 64%.