New Roads to Resource Recovery
An increased emphasis on resource recovery is driving innovation and development in sludge treatment processes
by Dr Ana Soares, Lecturer in Biological Engineering, Cranfield University
Treatment processes for sewage sludge (also called biosolids) are currently evolving at a very fast pace in response to current pressures, that can be ultimately summarized as the need to look for opportunities for enhanced resource recovery. Sewage sludge has a high content of organic carbon compounds (20-40% of total solids) and nutrients (nitrogen 1.5-4% of TS and phosphorus 0.5-2.8% of TS) but also heavy metals and trace pollutants, containing resources that can be recovered as energy (biogas, hydrogen syngas), nutrients, bio-diesel, bio-plastics, bio-sorbents, construction materials, heavy metals, proteins, enzymes, and so on.
Traditional methods for resource recovery from sludge are focused on energy production through biogas, using anaerobic digestion processes. The main goal of anaerobic digestion is to stabilize the sludge, i.e. reduce pathogen counts and reduce the volume for re-use as soil fertilizer or conditioner. Anaerobic digestion is able to capture the energy content from sludge as a biogas, which is usually 40% CO2 and 60% methane, but only 20-30% of the organic matter is actually mineralized, against the total estimated energy stored in sludge, i.e., 23,000-29,000 kJ/kg TS in primary sludge and 19,000-23,000 kJ/kg TS in activated sludge. The use of pre-treatment processes that promote fast hydrolysis is becoming more widely spread to enhance breakdown of the complex compounds into simpler molecules, resulting in higher methane production yields. Some examples include sludge disintegration through mechanical shear, ultrasound (e.g. Ultrasonic), a combination of temperature and pressure (eg: Cambi and BioThelys) or enzymes.
New research has also demonstrated the benefits of enriching anaerobic digesters with external carbon dioxide. A recent study completed at Cranfield University has established an increased methane production of 96-138% during the first 24 hours of digestion after adding CO2 gas to the process as a single pulse of up to 20 minutes. This process has the advantage of promoting biogenic carbon sequestration, reducing the greenhouse gas (GHG) emissions, at the same time as enhancing biogas production.
Other processes such as incineration and co-incineration, gasification, pyrolysis, wet air oxidation, supercritical wet oxidation and hydrothermal treatment also look into recovering energy from sludge. Incineration and co-incineration are presently focused on energy recovery as heat or electricity and have been implemented successfully in many sewage treatment works (STWs) world-wide. More recently gasification has been implemented in Europe with two plants in Balingen, Germany (2002) and Mannheim, Germany (2012) both supplied by Kopf Syngas, and one in the UK at Yorkshire Water’s Esholt STW supplied by Intervate.
Energy from gasification
In general, gasification is completed by processing dried sludge into ash (also called biochar) and combustible gases (syngas) at high temperatures (>1000 °C) under reduced oxygen concentrations. The energy production by the gasification is dependent on temperature, pressure and sludge characteristics (VS and TS content). The syngas produced can be used as gas turbine or a boiler fuel or, if the quality is high enough, can be used to replace natural gas. In same cases, dry feedstocks such as wood or green wastes might be added to the sludge to meet specific syngas quality production.
The resulting biochar has been demonstrated to be useful for a number of applications on sewage treatment works such as soil conditioner, adsorbent for tertiary water treatment, construction materials, for phosphorus recovery etc. although full-scale use is still limited.
Other emerging processes that have been implemented at full-scale are dedicated to phosphorus recovery. The main driver for the recovery/removal of phosphorus from sludge dewatering liquors and centrates is to avoid scaling. Formation of struvite and calcium phosphate occurs after changes in chemical and physical properties of the centrates such as temperature and pH, creating significant scaling problems in pipes, centrifuges, heat exchangers, etc. that will need to be fully replaced to get back into service. On the other side, phosphorus has been named the “disappearing nutrient” due to the finite rock reserves worldwide and there is also concern due to the unequal worldwide distribution of these reserves. Currently there are a number of full-scale struvite production plants world-wide, including one at Thames Water’s Slough STW and another currently being commissioned with Paques Phospaq at Severn Trent Stoke Bardolph STW.
A new option for struvite recovery is currently being researched at Cranfield University. Laboratory-scale experiments have demonstrated the benefits of using specific bacteria that can produce biologically induced struvite as part of their normal metabolic pathways in sludge dewatering liquors opening a completely new route to recover phosphorus and ammonia as struvite from wastewater. Biological crystal formation of phosphorus compounds (e.g.: struvite; magnesium phosphate, etc.) has been demonstrated to be a by-product of the metabolism of specific bacteria that can be found frequently in the environment. Current research is focused on building and operating a pilot-scale bioreactor to demonstrate the benefits of the process and understanding the bio-struvite production yields and growth conditions of B. antiquum in mixed cultures. This bacteria is capable of forming bio-struvite crystals at pHs between 6-8 reaching 250 µm in size.
Ammonia recovery can also take place through struvite production, but there are not many options available for ammonia recovery from sludge and centrates streams. Cranfield University has been developing ion exchange processes to remove and recover nutrients from liquors/wastewater. More specifically the project considers 2 different ion exchange media: a hybrid ion exchange media (HAIX) for P removal and Al 3+ ion exchange media (MesoLite) for ammonia removal. Recovery of the ammonia and phosphate will be achieved through the regeneration cycles as ammonia sulphate and calcium phosphate, which are fertiliser precursors. These technologies are being demonstrated at pilot-scale with the support from Yorkshire Water through the long-term strategic partnership with Cranfield University.
This article has aimed to highlight just a few possible options for resource recovery from sludge. Other exciting technologies that are being tested at pilot-scale such as recovery of heavy metals, production of bio-plastics, bio-fertilisers and bio-pesticides might reach full-scale implementation in the coming decade. Although relevant technologies for recovering resources from sewage sludge are being developed and demonstrated to be technically possible even at full-scale, the social and economical feasibility is still a major issue. The fact that products are being generated from sewage and sludge raises a number of regulation and social concerns that are difficult to address. More successful cases need to be made as examples so others can follow. Liaising with regulators, the general public and the creation of viable supply chains are key steps to ensure the success of these technologies.
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