8
Resource Recovery and Emerging Technologies
WRRFs no longer just treat water and generate waste as a by-product, but recover resources that are
embedded in wastewater, such as energy, renewable natural gas (biogas), nutrients, and clean water.
Added benefits of energy and biogas recovery are the reduction in quantity of solids disposed of in
landfills, pathogen reduction in biosolids, and reduction in fossil fuel extraction. Nutrients recovery
offsets the manufacture and application of synthetic fertilizers, while reducing nutrients discharged
to waterways.
There are a few proprietary technologies that can be used to recover dissolved phosphorus from
digested sludge and digester supernatant to produce a marketable mineral fertilizer. The first step of
these processes is biological or chemical phosphorus removal from wastewater to transfer phosphorus
to the sludge. Next, chemical treatment is used to form a phosphorus mineral, such as struvite, that can
be sold to generate revenue.
Biogas generated by anaerobic digesters contains approximately 60% methane. The remaining
components are carbon dioxide, water, and other impurities such as hydrogen sulfide and siloxanes.
Biogas can be treated to separate methane from the other constituents. The resulting product is
relatively pure methane that meets quality standards of pipeline natural gas, also called renewable
natural gas (RNG), which qualifies under the Renewable Identification Number (RIN) program when used
for vehicle fueling. Petroleum producers are obligated to meet a certain quota of RINs, so there is a
trading market for them, from which the WRRF can have an economic benefit. Economic incentives can
also be associated with recovering renewable energy from the WRRF, in the form of Renewable Energy
Certificates (RECs), which can be sold to create a supplemental revenue stream in addition to the
avoided cost for purchasing electricity that is generated with the recovered biogas.
Some promising new technologies are being researched to reduce energy use in systems that are
currently energy intensive. For example, shortcut nitrogen removal uses anaerobic ammonium oxidizing
(anammox) bacteria that convert ammonia to nitrogen gas without the requirement of additional air or
carbon. This system has application in sidestreams such as nitrogen-loaded anaerobic digester
supernatant and dewatering waste, as well as mainstream systems.
Post Aerobic Digestion (PAD) is the addition of an aerobic digester following the anaerobic digestion
process. PAD provides additional destruction of volatile solids, improving dewatering performance and
reducing the volume of disposed biosolids. It also can remove ammonia and organic sulphur compounds
(a source of odors). The PAD process increases annual aeration costs but eliminates the need for a
separate sidestream treatment process to manage nitrogen and can reduce some ortho-phosphorus by
forming struvite as the reactor pH increases.
Another emerging technology in digester enhancement is waste activated sludge (WAS) hydrolysis,
which is a process where the cell walls of the bacteria contained in WAS are ruptured (lysed). The
process can be accomplished by mechanical, thermal, chemical, or pressure-based means or a
combination of several of these methods. The process offers many advantages, such as increased solids
retention time (SRT) and reduced digester heating loads; improved digestibility, resulting in enhanced
biogas production; and improved dewaterability resulting in reduced volumes of disposed biosolids.