Municipal Sewage Sludge Composting in the Two-Stage System: The Role of Different Bulking Agents and Amendments

Abstract

This study assessed the effect of different lignocellulosic amendments and bulking agents on compost stability (based on a 4 day respiration activity test, AT4, and self-heating factor, SHF) and maturity (based on the nitrification index Initr and the ratio of C in humic acids, HA, to total organic carbon, TOC, in compost, CHA/TOC). With all feedstock compositions (FCs), the share of sewage sludge was 79% (wet mass). For FC1, wood chips (13.5%) and wheat straw (7.5%) were used as bulking agents and amendments; for FC2, instead of wood chips, energy willow was added; for FC3, pine bark (13.5%) and conifer sawdust (7.5%) were used. All FCs produced stable and mature compost; however, with FC2, the thermophilic phase last 3 days longer than with the other FCs. Moreover, an AT4 value below 10 g O2/kg dry mass (d.m.) was obtained the earliest with FC2 (after 45 days, ca. 15–20 days earlier than with other FCs). With FC2, Initr below 0.5 was obtained in ca. 60 days, 10 days earlier than with FC3 and 30 days earlier than with FC1. The highest net increases in HS (86.0 mg C/g organic matter (OM)) and HA (56.3 mg C/g OM) were also noted with FC2; with other FCs, the concentrations of these compounds were from 1.3- to 1.5-fold (HS) and from 1.4- to 1.9-fold (HA) lower. With FC2, the highest CHA/TOC (15.5%) was also noted, indicating that this compost contained the largest share of the most stable form of organic carbon. The rates of OM removal in the bioreactor ranged from 7.8 to 10.1 g/(kg d.m.·day). The rates of SH and HA formation ranged from 1.63 to 4.83 mg C/(g OM·day) and from 1.23 to 1.80 mg C/(g OM·day), respectively. This means that, through the choice of the amendments and bulking agents, the length of the composting time needed to obtain a stable and mature product can be controlled.

Functional and sequence-based metagenomics to uncover carbohydrate-degrading enzymes from composting samples

“…The significant differences obtained for these particular parameters between the TF and LP samples may be related with the presence of municipal sludges in the TF sample. Municipal sludges are rich in nitrogen and are characterized by high moisture content and low porosity (Kulikowska et al 2022 ). Both, the TF and GN samples, presented a high percentage of municipal sludges and moisture content (> 63%).…”

“…The mixture of green organic wastes with lignocellulosic residues (brown residues) may have contributed to the differences observed in the total carbon and nitrogen contents. Green wastes generally present higher nitrogen content, while brown residues are mainly rich in carbon (Hemati et al 2022 ; Kulikowska et al 2022 ). For the three samples, the C to N ratios did not differ significantly, being obtained values around 11.…”

“…The lower C to N ratios together with the high moisture contents may explain the longer periods, due to a slower degradation, to reach the typical temperatures of the thermophilic phase of the composting process. Usually, temperatures above 50 °C are reached after a week (Guo et al 2012 ; Awasthi et al 2014 ; Zhang et al 2020 ; Kulikowska et al 2022 ). Regarding the pH variations, the TF and GN samples presented values close to 7 that benefit the microbial activities responsible for the decomposition of organic matter to produce organic acids and inorganic salts.…”

Study on the Effect of Thermal Assisted Combined Plant-Based Biomass Conditioning on Dehydrated Sludge Bio-Drying

“…Therefore, this type of plant-based biomass is usually chosen as a conditioner to improve sludge dewatering and bio-drying performance [17,18]. Because the type and ratio of conditioning agents can affect the effectiveness of sludge bio-drying [6,[19][20][21][22], it is necessary to study the optimal conditioner for sludge bio-drying and optimize the ratio of conditioners to minimize the moisture content of drying products and retain their calorific value. This is crucial for further promoting sludge bio-drying technology.…”