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As stated in WEF’s position statement, water resource recovery facilities have the potential to be energy neutral or even net energy producers through holistic energy management approaches, incorporating conservation practices, and generating renewable energy through treatment of their by-products, such as biosolids. Solids treatment provides the greatest potential for energy recovery and production, with the chemical energy embedded in biosolids greater than the energy needed for treatment.
Enabling Energy Recovery
Driven by rising energy costs and sustainability concerns, utilities are recovering previously wasted resources – flared biogas and waste heat – to increase their energy self-sufficiency. A variety of well-proven energy recovery technologies is available for onsite energy production, and innovative technologies are poised to expand the options.
The energy in wastewater exists in three forms: thermal energy, hydraulic energy, and chemical or calorific energy. There are many opportunities to convert the chemical energy in solids to a useable form (heat or fuel) through biological or thermal processes. Energy recovery options range from mature, well-established systems, such as anaerobic digestion (AD) and incineration, to emerging technologies, such as Supercritical Water Oxidation (SCWO) and hydrothermal gasification. These options fall into two main categories: bioconversion and thermal conversion.
Biogas production through AD is limited to conversion of the readily biodegradable portion of the solids. To overcome this limitation, and thus maximize biogas production, co-digestion and pretreatment processes have become rapidly growing practices in recent years.
The biogas generated by AD systems is an extremely versatile fuel and can replace natural gas for heating and power generation needs. Heat recovery is by far the most common use of biogas, with a majority of facilities using biogas in boilers or recovering heat from CHP to heat digesters and/or buildings. The primary use of biogas at most facilities is digester heating. With increasing fuel costs and sustainability concerns, many plants are trying to maximize the use of biogas in place of purchased energy.
Combined Heat and Power (CHP)
Increasingly, plants are using biogas in CHP systems to generate electricity from the biogas. Internal combustion (IC) engines are the most widely used CHP technology. Combustion gas turbines are often a good fit for the largest WRRFs. Like IC engines, combustion gas turbines are a reliable, well-proven technology.
As the name suggests, a microturbine is a much smaller version of a combustion gas turbine. Microturbines are often a good fit for smaller WRRFs with anaerobic digestion. Microturbines have become the second most widely used CHP technology at WRRFs due to their small capacity and clean emissions.
Fuel cells are unique in that they do not combust biogas to produce power and heat. Instead, fuel cells convert chemical energy to electricity using electrochemical reactions. Their benefits include high electric efficiency and extremely clean exhaust emissions.
Biogas Upgrading
Currently, only 1% of the biogas beneficially used is upgraded to natural gas quality for injection into the natural gas transmission system. Biogas is also upgraded to CNG for use as fuel for CNG vehicles.
Pipeline Injection
Pipeline quality biogas has extremely low concentrations of contaminants and must be compressed to match the natural gas transmission line pressure.
CNG or LNG Vehicle Fuel
Biogas can be upgraded to displace CNG or liquid natural gas (LNG) in vehicles capable of using these fuels. In Europe, upgrading biogas to fuel vehicular fleets is an established practice. In the U.S., there are only a few installations. Purity requirements for vehicular fuel are lower than those for pipeline injection. The biggest barriers to CNG or LNG conversion are the lack of a widespread infrastructure for gas filling stations and the cost of vehicle conversion for CNG or LNG use.
Use of Biogas in Industrial Processes
There are several examples of efficient use of biogas by industries sited in proximity to WRRFs. In these situations, biogas that is untreated or minimally treated is provided to an industrial facility that utilizes the gas in its processes.
Thermal Conversion
In contrast to biological conversion (anaerobic digestion), thermal conversion of wastewater solids can make use of all of the chemical energy embedded in the solids, regardless of degradation potential.
Off-site Co-combustion
Instead of incinerating biosolids at the treatment plant, biosolids can be used to supplement or replace coal in cement kilns and coal fired power plants.
Gasification
Gasification is the thermal conversion of carbonaceous biomass into syngas.
Thermal Oxidation
Thermal oxidation (incineration) is the most established biosolids thermal conversion technology and has been used since the 1930s, and has been practiced in the wastewater sector mainly as a volume reduction/sterilization method of biosolids management. Looking toward the future, municipal utilities are actively looking at energy recovery and production. Thermal oxidation involves the complete oxidation of all organic material by applying heat in the presence of excess oxygen. The volatile fraction of the feed material is converted to hot flue gases, while the non-volatile or inert fraction becomes ash. Thermal energy is often recovered from the high temperature flue gas and may be used to generate electricity using a steam turbine.
Pyrolysis
Pyrolysis is the thermal conversion of carbonaceous biomass in the absence of oxygen.
Thermal Conversion in Supercritical Water
The concept of applying thermal conversion to liquids is attractive, since it eliminates the need for moisture removal and therefore reduces process energy requirements. Supercritical water (SCW) is a state in which water behaves as both a gas and a liquid and occurs at high temperatures (greater than 374°C) and pressure (greater than 221 bar).
For additional information, see biogasdata.org.
WEF continues to establish conditions that promote accelerated development and implementation of innovative technologies and approaches, and is collaborating with water sector partners in a call to action to accelerate resource recovery (WEF Strategic Plan, 2015).
