By Robert Hacking
Challenges facing wastewater treatment utilities today often seem greater than ever. Communities are increasingly connected to broad themes of sustainability, including resource recovery, reduced carbon emissions and carbon footprint, and lower operating costs. Energy neutral wastewater treatment is one area where operators can address growing sustainability expectations, while not risking any economic viability.
Energy can account for 30% of a wastewater treatment plant’s total operation and maintenance budget. This percentage will grow as the demand for advanced treatment and its inherent energy consumption increases as well. In an effort to combat this, operators are embracing a shift from wastewater treatment to resource recovery. This involves harnessing the energy content inherent in wastewater, which is two to four times the amount required to treat it.
With this mindset, plants can go from energy consumers to energy producers, convert what was once a discharge into a reusable resource, and recover and utilize nutrients like nitrogen and phosphorus.
There are four fundamental components that comprise an energy neutral wastewater treatment plant: enhanced primary treatment, biological treatment, advanced sludge treatment, and energy recovery.
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Advanced anaerobic digestion is a type of advanced sludge treatment in which bacteria, in the absence of oxygen, break down biosolids created during the biological treatment process. The valuable byproducts, including methane, can be combusted for heating or processed through a Jenbacher gas engine to produce electricity and heat.Table 1: The fundamental building blocks that comprise an energy-neutral wastewater treatment plant; the highlighted row indicates where biological hydrolysis is applicable.
|Component||Description / Benefit||Example|
|Enhanced primary treatment||Single step for separation, thickening, and optional dewatering of primary solids; can increase the amount of diversion of digestible solids to anaerobic digestion||GE’s LEAPprimary*|
|Biological treatment||Removes nutrients and organics from sewage; a low-energy option boasts 4x greater efficiency than conventional fine bubble aeration||GE’s ZeeLung*
membrane-aerated biofilm reactor (MABR)
|Advanced sludge treatment||Organics shunted from biological treatment are processed through advanced anaerobic digestion, converting sludge into biogas that can then be converted to electricity & biosolids; 20% to 30% higher biogas yield than conventional anaerobic digestion; enables the treatment of sludge in reduced digester retention times, allowing for optimized digester volume to treat more sludge, and/or external wastes||Biological hydrolysis using Monsal* advanced
|Energy recovery||Biogas produced during advanced anaerobic digestion is converted into electricity and heat by gas engines or alternatively can be upgraded to biomethane and injected into natural gas pipelines||Jenbacher* gas engines|
The advanced anaerobic digestion process can be configured in many different ways. One of these is biological hydrolysis, a non-invasive solution that is installed upfront of existing anaerobic digestion infrastructure to enable maximum digester efficiency.
A biological hydrolysis system consists of six serial reactor vessels, whereby sludge is heated to 42°C in the first reactor. Thermal energy is typically drawn from a plant’s hot water loop, with heat provided by a biogas boiler or a combined heat and power engine if available.
The reactors operate in a semi-continuous reverse cascade batch system, where sludge is batched once per hour from reactor six (R6) to the digester, then R5 to R6, R4 to R5, and so on to R1 and R2. Following the final transfer, a fresh batch of sludge is transferred to R1. (See Figure 1)
Total hydraulic retention time (HRT) is designed to be approximately three days. The ultimate goal is to transfer sludge to the anaerobic digester prior to significant methanogenesis. With each batch, only a portion of the reactor vessel is transferred forward, leaving a concentration of enzymes behind. The sludge is only heated in the initial reactor, but progressively cools to between 35°C and 40°C prior to entering the methanogenic digester.
Mixing in the reactors is powered by unconfined gas recirculation. Biogas is drawn from the head space of one reactor, compressed, and injected into the base of the next reactor. Each of the reactors is completely mixed, but short circulating is minimized due to the system’s operation in reverse cascading batch flow. This ensures that all sludge entering the R1 vessel spends the majority of the design HRT within the biological hydrolysis system, prior to entering the digester.
Batch transfer through six reactor vessels essentially represents a plug flow across the entire process. Biological hydrolysis, combined with the system’s ideal retention time and operational strategy, promotes acidification. Sludge exiting the system is expected to have high volatile fatty acid (VFA) concentrations, which conditions sludge for biogas conversion downstream in the anaerobic digester.
In addition to operating at 42°C, biological hydrolysis systems include Monsal 55, an enhanced treatment step that incorporates pasteurization. The technology was developed by Monsal Limited, a company that was acquired by GE Water & Process Technologies in 2014.
With Monsal 55, reactors R4 through R6 function at an elevated temperature of 55°C. Operation is a true batch hold process, whereby vessels R5 and R6 operate in parallel and the entire vessel contents are held to be safely pasteurized.
Biological hydrolysis systems can accommodate feed sludge with a dry solids concentration of up to 8%. Sludge exiting the system still has a 6% to 8% dry solids concentration. However, since the natural hydrolysis process reduces viscosity drastically, it has characteristics similar to raw sludge at 2% to 3% dry solids concentration.
The biological hydrolysis process uses reactor vessels, heat exchangers and gas compressors, for mixing and transfer. The steel tanks used for heating are epoxy or glass lined and insulated and cladded to ensure energy efficiency. The process offers an economical alternative to building more digester volume and can even free up capacity for other organic solids.
The technology’s footprint is compact, which means it can be retrofitted into existing facilities in a non-invasive way. Plants can continue to utilize existing digester mixing, as biological hydrolysis alters sludge microstructure, not the manner in which it is processed.
Biological hydrolysis enables an increase in the digester organic loading rate, thereby enhancing biogas production and reducing biosolids mass. By processing the biogas yield through a combined heat and power gas engine, operators can produce electricity using the biogas byproduct as well as recapture all required heat.Table 2: In just five short years, a sewage plant is transformed from an energy consumer to an energy producer. Biological hydrolysis was a first and crucial step in this process.
|Pre-year 2007||2007||2009||Beginning |
|% sludge treated||51%||86%||100%||100%||100% + FW|
|Digester volume||16,400 m³||16,400 m³||25,200 m³||20,800 m³||25,200 m³|
|Electricity produced||1.9 MWe||2.9 MWe||4.0 MWe||4.0 MWe||+5.75 MWe|
Theory in practice
Since 2002, 12 Monsal biological hydrolysis plants have been commissioned, mainly in the United Kingdom. The plants range in size from 4,500 to 40,000 tonnes of dry solids sludge feed (tds) per year. These installations have allowed plant owners to maximize indigenous sludge digestion as well as import additional sludge volumes and/or other organic wastes to treat in existing digester infrastructure.
Bristol Sewage Treatment Facility is a 300 ML per day wastewater treatment plant. Prior to incorporating biological hydrolysis, the plant had primary clarifiers and sequential batch reactors. The plant only treated 51% of its sludge for digestion, with the remainder being lime stabilized.
The plant’s desire to become a regional sludge centre and to move towards energy neutral wastewater treatment operation led them to biological hydrolysis. The addition of this advanced process increased sludge treatment from 51% to 86% of both indigenous and imported sludge. Also, it increased power production from 1.9 MWe to 2.9 MWe.
Following biological hydrolysis incorporation, biosolids holding tanks were repurposed to digesters to expand sludge capacity. This enabled 100% of the sludge to be treated. With volume exceeding need, two of the digesters were reallocated for digestion of imported biowaste feed. This ability to digest sludge and food waste at a co-located facility was timed perfectly, with a community ban on organics to landfills.
The advanced anaerobic digestion process holds an abundance of opportunity for those willing to take the proactive step to incorporate the technology. For example, the Great Billing Water Recycling Centre, also in the UK, shows how biological hydrolysis can lead to energy-neutral wastewater operations. Commissioned in 2009, their Monsal 55 system enabled a 300% increase in biogas production and 4.2 MWe of renewable electricity generation. This was achieved through biological hydrolysis, which more than tripled the sludge treated through the digestion portion of the plant.
Projects such as these show that biological hydrolysis can transform plants from energy consumers to energy producers.
Robert Hacking is with GE Water and Process Technologies (Canada). This article appears in ES&E Magazine’s August 2016 issue.