Hunter Water Corporation The construction of Biosolids storage areas at various Hunter Water Wastewater Treatment Works (WWTW) across the Hunter Region, including: Belmont, Morpeth, Boulder Bay, Raymond Terrace, Edgeworth, Toronto, Kurri

Abstract

The high moisture content of biosolid from a wastewater treatment plant limits its use for agriculture and energy applications. This limitation could be obviated by hydrothermal carbonization, which requires less energy compared to other thermochemical treatment processes, and results in stabilized solid hydrochar product. The present study examined this option by hydrothermally treating the biosolid at three temperatures (180, 200 and 220 °C) for 30 min, and at 200 °C for 15, 30 and 60 min. An increase of 50% in the heating value of the biosolid was obtained after this carbonization. A reduction in the nitrogen concentration in hydrochar was noted with an increase in phosphorus concentration, but potassium concentration remained largely unchanged. Additionally, the carbon to nitrogen ratio in the hydrochar product was higher than the biosolid that makes it suitable for agriculture applications. The chemical oxygen demand of the process water was in the range of 83,000 to 96,000 mg/L. The study thus provides insight into high-value products that can be generated by the hydrothermal carbonization of biosolids.

Keywords: 

hydrothermal carbonization; biosolid; hydrochar; process water; biogas

1. Introduction

The terms sewage sludge and biosolids represent the same semi-liquid end product of wastewater treatment. Biosolid (or sewage sludge) is treated to make it suitable for land applications [1]. In Canada, 660,000 metric tons of dry biosolid is produced annually [2]. Its management represents 50% of the overall operating cost of a wastewater treatment plant. Only half of the total biosolid is utilized in land while another half is disposed of. Disposal or management options for biosolids include incineration and landfills. These options do not facilitate recovery of the valuable organics in biosolid and adversely affect the environment and human health [3]. Concerns about contamination of land with harmful biological and chemical species in biosolids and other practical limitations prevent wide scale use of biosolids in agriculture. One of the practical issues is its high moisture content, which makes transportation difficult [4]. Such is the case of the wastewater treatment plant of Charlottetown in the province of Prince Edward Island, Canada. This plant treats approximately 7 million cubic meters of wastewater annually producing roughly 3700 tonnes of biosolid. Presently, the sewage sludge is pre-pasteurized, anaerobically digested and dewatered producing biogas and class-A biosolid. The biosolid is then taken away with a tipping fee of $35/ton for disposal. Drying could be an option to increase the dry matter content, but it is an energy intensive process. For biomass with such high moisture, hydrothermal carbonization could be a preferable option. Subcritical hydrothermal carbonization (HTC) is typically carried out at a temperature of 180–220 °C, with pressure in the range of 2.5–5 MPa, and for a residence time of 15–120 min in an inert or oxygen-free environment [5]. Since the hydrothermal treatment process does not involve evaporation of water, the energy required for the process is low [6,7]. Lucian et al. [8] found the temperature of 220 °C and 1 h residence time to be the most favorable condition for hydrothermal carbonization of waste biomass (grape marc). For that condition, they found the plant efficiency to be 78%, with a specific thermal energy consumption of 1.17 kWh per kilogram of hydrochar and specific electric consumption of 0.16 kWh per kilogram of hydrochar. They found the production cost of pelletized hydrochar to be 157 €/ton hydrochar and 200 €/ton hydrochar was the break-even value for a plant with a payback period of 10 years. Additionally, this process could produce a more stable hydrochar from which it is easier to remove water [7]. The process also produces process water containing organic compounds with higher chemical oxygen demand, which can be anaerobically digested to produce biogas. Biogas, thus produced, could supplement the energy required for the process. Apart from anaerobic digestion, process water can be used as a liquid fertilizer. Sun et al. [9] carried out the hydrothermal process at 180, 200 and 220 °C to extract the nutrients from sewage sludge and studied nitrogen, potassium, phosphorus and heavy metal content in the liquid products obtained after hydrothermal process. The extracted liquid products were experimented as a potential fertilizer in Komatsuna plant cultivation in a small-scale Results showed improvement in the crop yield. In recent times, only a few experiments have been performed on hydrothermal carbonization (HTC) of sewage sludge. For example, in one HTC at 200 °C, 2.1 MPa for 1–6 h, the hydrochar yield was 74–81% and depended on the carbonization time [10]. Another HTC on sewage sludge was carried out at 200 °C, 1.5 MPa and residence time of 4, 7, 10, and 12 h [11]. The higher heating value of the product was 14.73 MJ/kg for residence time of 10 h [10]. Most of the works on HTC has been carried out for longer residence times. Parshetti et al. [12] studied the co-combustion of sludge char from urban sewage sludge with low rank Indonesian coal as well as hydrothermally upgraded coal at 250 °C, 8–10 MPa and 15 min reaction time. The results showed that combining sludge char with low rank Indonesian coal and hydrothermally upgraded coal improved the quality of combustion emission and the HTC process is suitable for energy generation as well as co-combustion.

A feasibility study to compare hydrothermal carbonization (HTC) with conventional drying methods for sewage sludge found that HTC required less input energy and it is dependent on the dry mass content of the product [6]. Similar results were obtained for hydrothermal carbonization of sewage sludge with waste biomass as the energy balance shows low energy input as compared to the output energy [13]. There is thus a need for systematic study of this promising option for biosolid treatment. The present work investigated HTC of biosolids from a commercial sewage treatment plant and analyzed the overall system efficiency based on the first law of thermodynamics. The results of hydrothermal carbonization of biosolids at different operating conditions are presented and discussed in this paper.