Enhancing biomethane production and phosphorus recovery from aerobic granular sludge through a thermal alkali pretreatment

Abstract

The development of aerobic granular sludge (AGS) technology offers a sustainable and efficient alternative for biological nutrient removal (BNR) systems. The anaerobic/anoxic/aerobic (A2O) process, the most widely used BNR process that removes carbon (C), nitrogen (N), and phosphorus (P), consists of anaerobic-anoxic-aerobic compartments with various redox potentials and a separate settling tank for liquid-solid separation. The BNR system requires a large footprint for multiple process units, including aeration and settling tanks, as well as high energy consumption and capital costs. The granular sludge is distinct from activated sludge with regard to compactness, particle size, settling velocity, matrix of extracellular polymer substances (EPS) and structure of microbial communities. The implementation of AGS technology can result in a significant reduction in land requirements (up to 75%), energy consumption (up to 50%), and capital and operational costs (up to 50%). Anaerobic digestion (AD) of the excess sludge from AGS process is an environmentally friendly and effective technology that recovers energy in the form of heat and electricity and stabilizes sludge. Various pretreatment methods have been employed to increase methane production and sludge digestibility in AD. These methods break down the complex structure of sludge, promote hydrolysis, and degrade organics, making them more accessible to microorganisms responsible for the digestion process. One widely used pretreatment method is thermal pretreatment, which utilizes temperature to break down organic matter and increase biogas production. Another effective pretreatment method is alkaline pretreatment, which involves adjusting the pH of the sludge by using alkaline substances. A combination of low-temperature thermal and alkaline pretreatment, known as low-temperature thermal-alkaline pretreatment, is also effective in improving the AD process. During AD, a significant amount of P is released into the supernatant, leading to higher phosphate concentrations in the effluent. Polyphosphate-accumulating organisms (PAOs) present in aerobic granules have the ability to take up phosphate from the supernatant. Moreover, struvite precipitation is considered as an ideal technique for P recovery due to its unique properties. Struvite is a slow-release fertilizer that releases nutrients at a slow rate, providing sustained nourishment to plants. The application of struvite precipitation enables the recovery of P from wastewater and its conversion into a valuable resource that can be used as slow-release fertilizer in agriculture. The aim of this study was to investigate the treatment performance of the AGS process and resource recovery potential (energy and P) of the granular sludge from AGS process. The laboratory scale AGS system was operated under steady-state conditions for 56 days. A low-temperature thermal-alkali (LTTA) pretreatment method was applied to the excess granular sludge from AGS process at 100 C and various pH values (9, 10, 11, and 12) in order to enhance methane production. To quantify biomethane production, a biomethane potential (BMP) test was performed on the LTTA pretreated sludge samples. The study also aimed to determine the P recovery potential at different magnesium/phosphorus (Mg/P) molar ratios (1:1, 1.5:1, and 2:1). A mass balance for chemical oxygen demand (COD) and economic analysis were conducted. According to the results of this study, high COD removal efficiency over 80% was obtained. Total nitrogen (TN) and total phosphorus (TP) removal efficiencies were approximately 73.5 ± 1.3% and 69.9 ± 1.1%, respectively. The highest methane yield with a value of 216 ± 22 mL CH4/g VS was observed in the LTTA pretreated sludge from the AGS process at pH 10 (S3), which was approximately 1.3 times higher compared of the sludge from the AGS system without any pretreatment method (S0). This finding highlights the significant impact of the pretreatment method on methane production. At a Mg/P molar ratio of 2.0:1.0 and pH 10, a maximum P recovery rate of 92.6 ± 0.8% was obtained. A COD mass balance was performed for S3, which showed that 13.3% of the COD could be converted to methane gas. Additionally, the unit cost and benefit for production of struvite was determined to be 3.40 $/ton sludge and 12.83 $/ton sludge, respectively. This study highlighted the role of the LTTA pretreatment method on resource recovery from the excess sludge of AGS process. The application of thermal-alkali pretreatment presents a promisingly cost-effective and sustainable approach, providing environmental benefits and enhancing energy and phosphorus recovery from excess sludge within the AGS process.