Sewage Sludge Management at District Level: Reduction and Nutrients Recovery via Hydrothermal Carbonization

Abstract

In this study, two scenarios of a municipal wastewater treatment plant (WWTP) are presented, which include the integration of the hydrothermal carbonization (HTC) process into the sludge line as a post-treatment of the anaerobic digestion (AD) process. The objective of the simulation is to investigate the performances of AD + HTC treatment to reduce sludge production and improve nutrient and energy recovery. For this purpose, the scheme of an under-construction WWTP was considered, named Trento 3 (Trento, Italy) and with a treatment capacity of 300,000 PE. In the first scenario, the HTC process was fed with thickened sludge from the Trento 3 WWTP, while in the second scenario, dewatered sludge from other local WWTPs was also used as feedstock for the HTC process. Both scenarios allowed to obtain a considerable sludge reduction ranging from 70 to 75% with a notably increase in the biogas production up to 47%, due to the recycling of HTC liquor (HTCL) to the anaerobic digester. Considering nutrients recovery, all the phosphorus and nitrogen present in the HTCL could be used for struvite precipitation with an average yearly gain of 1 million euros. Moreover, the introduction of HTC in the Trento 3 WWTP could allow a reduction in the sludge management costs of up to 2 M€/year. Graphical Abstract

Technoeconomic assessment comparison of batch and continuous hydrothermal carbonization of waste corn stover into advanced biorefinery feedstock

“…Here, the makeup water requirement for both batch and continuous HTC reactor was assumed to be collected from municipal wastewater, which not only eliminates the cost of the water utility but also positively affects the product quality. 43 For case I, an additional 1.5 h downtime was considered to complete each batch cycle (12 cycles day −1 ). For case II, the reactor was run in a continuous fashion, assuming each reaction cycle completes in 0.5 h of space time as the feed goes along the length of the reactor.…”

“…The waste corn stover at 69 672 kg day ‐ 1 (12.8% of air classification throughput) was mixed with 335 572 kg day −1 makeup water to maintain 1:5 (w/w) dry solid‐to‐water ratio prior feeding to the stirred‐tank batch reactor (case I) and continuous‐stirred tank reactor (case II). Here, the makeup water requirement for both batch and continuous HTC reactor was assumed to be collected from municipal wastewater, which not only eliminates the cost of the water utility but also positively affects the product quality 43 …”

Mixing Due to Natural Convection in a Hydrothermal Reactor: A CFD Study

“…Hydrothermal treatments such as HTC and HTL also produce an aqueous phase with a high organic load which must be managed: this is another reason that identifies as ideal the location of hydrothermal plants inside WWTPs. Different configurations can be implemented: a thermal hydrolysis pretreatment applied to sewage sludge to increase its biogas production in subsequent anaerobic digestion, 41 HTC of digested sludge to reduce its volume and eventually recover nutrients, 42 a combination of the two aforementioned configurations. 43 These considerations highlight the importance of implementing CFD models capable of dealing with slurries and sludges during their treatment in hydrothermal reactors.…”

Phosphorus recovery from hydrothermal carbonization of organic waste: a review

“…Similarly, Scrinzi et al . (2022) proposed an upgrade of HTC of sewage sludge to a full industrial scale while considering nutrient recovery, and their proposition revolved around the recovery of 82.5% of P from HTC liquid product through struvite precipitation 119 …”

“…(2022) proposed an upgrade of HTC of sewage sludge to a full industrial scale while considering nutrient recovery, and their proposition revolved around the recovery of 82.5% of P from HTC liquid product through struvite precipitation. 119 The feasibility of P-recovery at large scales lies in the simplicity of the techniques adopted, whether on the hydrochar or the liquid product. The lack of complications in the upgrade from laboratory to industrial scale, coupled with its high recovery rates, summarize the feasibility of P-recovery from HTC of organic waste at the industrial scale.…”