Authors: Tom O’Reilly, regional business manager, Black & Veatch Ireland; John Fitzgerald, chief engineer, Sustainable Energy Solutions, Black & Veatch Ireland; and John Tattersall, global director of water technology, Black & Veatch Incorporating enhanced digestion processes into both new and retrofit wastewater treatment infrastructure projects may at first seem a niche technical concern of the wastewater sector. Indeed, it offers significant benefits to that sector: increased capacity of existing works, a byproduct safe for use as fertiliser, and the possibility for energy recovery. However, the latter attribute has much broader implications for Irish energy policy. It is particularly relevant to the government’s Bioenergy Plan which was agreed in July 2015. The core of this ambitious plan includes the introduction of a Renewable Heat Incentive (RHI) in 2016 and the continuity of the Renewable Energy Feed in Tariff (REFIT 3) relating to biomass. These incentives underpin the business case for biomass projects, including energy recovery through anaerobic digestion (AD) linked to wastewater infrastructure and the potential exploitation of renewable heat in on-site combined heat and power (CHP) plants.

Bioenergy Plan looks to future in terms of R&D

Furthermore, the Bioenergy Plan also looks to the future in terms of research and development and “the need to develop efficient ways to harness that resource, such as through the generation of electricity by AD plants”. Actions 11 to 13 of the plan emphasise the potential for leveraging Ireland’s significant research and development capability in this area, from basic research to pre-commercial demonstration. Action 13 specifically highlights that a detailed economic assessment of biogas and biomethane will be undertaken to identify the energy sectors where they can be cost-effectively deployed. The potential for innovations in the wastewater sector are therefore very promising in an Irish context. The possibilities of such plants playing an enhanced role in the overall energy infrastructure of Ireland were highlighted earlier this year in a previous issue of the Engineers Journal (April 7, 2015, ‘Greening the Gas Grid’), where Professor Jerry Murphy highlighted the opportunity of using anaerobic digestion alongside hydrogen gasification to enable the conversion of surplus wind generated electricity to green gas while supporting the stability of the electricity grid. This promising area of research and demonstration has the potential to connect key national engineering challenges in Ireland:
  • The need for investment and efficiency in water infrastructure;
  • Increased levels of biomass energy to meet renewable targets;
  • Enhancing electricity grid stability while integrating intermittent renewable generation.
[caption id="attachment_26707" align="alignright" width="300"]aaacrap The HPH facility at Colchester WRC with – from left to right - heating, pasteurisation and hydrolysis tanks[/caption] This raises the prospect of wastewater infrastructure having a key role in the demonstration of Irish technologies to cost-effectively exploit AD and innovative gas-to-grid solutions.

Thermal hydrolysis is one enhanced digestion process of growing significance

Thermal hydrolysis (THP) is one enhanced digestion process of growing significance in the wastewater sector. THP is a high-pressure process that typically heats waste active sludge (WAS) or a combination of WAS plus primary sludge for a short period of time to high temperature and pressure. The high temperature and the rapid decrease in pressure during depressurisation ruptures cells in the WAS and break down extracellular polymer, making more material bio-available for anaerobic digestion. This makes the sludge more digestible for microbes which results in significant improvements in digester loading and performance, opening up possibilities for improved energy and resource recovery from biosolids. The benefits are summarised in Table 1. THP technology was commercially developed by Cambi™ approximately 20 years ago. The Cambi system was used by Black & Veatch on the Dublin Bay project; it should be noted, however, that other proven advanced digestion systems are available. In the Cambi process, sludge is fed to a pre-heating vessel, the pulper, at a concentration of 16.5 per cent dry solids and is heated using steam recovered from the flash tank. Preheated sludge is fed to one of the reactors, heated via steam injection to 165oC, and held for 20 to 30 minutes. In addition to providing the required retention time for hydrolysis, the process meets the time-temperature criterion for Class A Biosolids, under the 40 US Code of Federal Regulations Part 503. Class A biosolids contain no detectible levels of pathogens.

Following the reaction period, sludge from the reactor is discharged to the flash tank

Following the reaction period, sludge from the reactor is discharged to the flash tank. During discharge to the flash tank, the sludge is rapidly depressurised. This causes cell rupture and the release of flash steam, which is recovered to the pulper. From the flash tank, hydrolysed sludge is fed continuously to anaerobic digestion via a cooling system which is required to reduce the sludge temperature from approximately 100oC or higher to the 40oC required for digestion. The most common current configuration for THP is to use it upstream of anaerobic digestion, although it can be used in other configurations. A thermal hydrolysis and digestion facility at United Utilities’ wastewater treatment works in Davyhulme, Manchester, has increased the site’s biosolids processing capability from 39,000 tonnes dry solids (tDS) per year to a maximum capacity of 121,000 tDS per year (with an average of 91,000 tDS per year). The new plant provides United Utilities with a central biosolids processing facility and allows for import of dewatered biosolids cake from seven satellite sites which previously used lime treatment. The increase in capacity means that Davyhulme is now capable of processing biosolids equivalent to a population of approximately 4.4 million people. The engineer, procure, construct (EPC) contract was undertaken by Black & Veatch. The THP at Davyhulme processes biosolids via two routes:
  • Liquid biosolids from Davyhulme treatment works is dewatered on site to approximately 25 per cent dry solids using four decanter centrifuges and fed to a storage silo which feeds the THP;
  • Imported cake is brought into site from the seven satellite sites via two cake reception facilities. The sludge is fed to the THP from two sludge storage silos.
There are four thermal hydrolysis streams feeding eight digesters (two digesters per stream). Each THP stream can be fed with dewatered biosolids from Davyhulme, imported biosolids or a combination of the two. Prior to being fed to the digesters, sludge is cooled to approximately 40oC.

Biosolids displaced from digesters flow by gravity into a degassing tank

Biosolids displaced from the digesters flow by gravity into a degassing tank where air is injected to inhibit methanogenesis. From the degassing tank, sludge is transferred to digested sludge storage tanks. Biogas from the digesters is balanced using two 9,000 cubic metre (m3) double membrane gas holders. The gas is pumped via a gas clean-up system to feed five CHP engines and three combination boilers. The combination boilers are capable of raising steam for the THP using a combination of exhaust waste heat (from the CHP engines) and biogas burned in the fired section of each boiler. Heat is recovered from the CHP engine jacket cooling systems and is used to pre-heat final effluent dilution water used to dilute sludge cake being fed to the THP and boiler feed water. Electricity is generated from the CHP engines and raises revenue through the UK’s Renewable Obligations Credit (ROC) system. Owned and maintained by Anglian Water, Colchester wastewater recycling centre (WRC) treats flows from 135,000 homes and businesses in and around Colchester, Essex. The existing WRC comprised a conventional digestion process and served as a sludge reception facility for liquid and cake imports from other Anglian Water satellite sites. The facility was recently upgraded as an enhanced digestion treatment centre with capacity for 14,526 tDS per annum. The facility’s increased capacity provides for all indigenous sludges from the works, expanded liquid sludge imports and a new cake import processing capability. The various sludges are blended, pasteurised, digested and finally dewatered to cake, ready for transportation off-site for use as an enhanced quality agricultural product. As principal EPC partner, Black & Veatch was selected to design and manage construction of the new treatment process. The work comprised modifications to the existing primary and imported sludge plant; enhanced tanker imports facility; new surface activated sludge (SAS) treatment; new cake reception facility; heating, pasteurisation and hydrolysis (HpH) process; new digesters and associated biogas holder; post-digestion dewatering and return liquors treatment facility. HpH is a unique advanced digestion process, developed, trialled, and up-scaled to full operational capacity by Anglian Water. The three-stage system performs the following functions:
  • pre-heats the blended feed sludges prior to pasteurisation;
  • pasteurises the sludge and achieve an ‘enhanced’ sludge product and rapid pathogen kill;
  • pre-conditions (hydrolyses) the digester feedstock for optimum biogas production.
Colchester’s HpH is in full operation and on target to treat 14,526 tDS per annum. The system comprises four reactors: one for pre-heating, followed by two for pasteurisation and one for hydrolysis. The system operates an automated batching arrangement centring on the two (280m3 each) pasteurisation tanks. While one pasteurisation tank is filling and heating, the other is holding at temperature and pasteurising its batch for at least five hours. After its holding period the batch is fed forward to the (1,115m3) hydrolysis tank and the cycle is repeated. The initial pre-heating stage utilises a circulating hot water loop (80oC) fed from the heat recovery systems of two CHP engines. Additional heat is added as required from two steam boilers which also utilise heat recovery from the CHP engines’ flue gases to create steam used by the process.

The pre-heating process raises the sludge temperature to 42oC

The pre-heating process raises the sludge temperature to 42oC at which time it is ready to feed the pasteurisation stage. This stage utilises direct steam injection to raise its temperature to 57oC. The hot water and steam systems are designed to harvest all heating needs from the CHP power generation process. If the CHP is out of service the boilers may be fuelled by burning surplus biogas, or if necessary, by diesel oil from the new storage tank. Following the pasteurisation stage the batch is discharged to the hydrolysis tank. The sludge is then transferred via a heat exchanger where final effluent is used to cool the flow to less than 38oC ahead of being slowly pumped forward to the digesters. Two new mesophilic anaerobic digesters, with a working volume of 3,802m3 each, receive the HpH treated sludge and hold it for a minimum period of 14 days. Biogas evolved from the digestion process is passed from the digesters to an existing (1,500m3) gas holder which feeds the site’s CHP engines. The aim of the CHP installation is to provide power generated by biogas from the enhanced digestion process. Two CHP engines rated at 1.2 MW each, provide sufficient power and heat to run the treatment process, but also allow the site to export surplus power to the grid. All power generated by this process is subject to ROC accreditation which generates an additional source of revenue. Total output from both engines is 2.4 MW/h; of this approximately 1.3 MW/h is consumed on site with the remainder taken up by the grid. Table 1 – Impacts of THP
Consideration Typical Improvement Achieved
Digester loading rate More than double the digester loading rate for conventional digestion.
Biogas production 25% to 30% additional biogas.
Overall energy production Site specific, but generally the additional energy required for steam is offset by energy recovered from increased biogas production
Volatile solids destruction (VSR) 60% VSR compared to around 45% to 50% VSR with conventional digestion.
Dewaterability Dependent on application a 30% dry solids in cake dewatered using centrifuges or belt filter presses (compared with around 23% to 25% dry solids for conventionally digested sludge)
Reduction in wet cake load out Up to 25% or 30% reduction in wet sludge volume for disposal (resulting from a combination of the additional VSR and the improved dewaterability).
Biosolids product Meets requirements for Class A biosolids under the time-temperature criterion.