Page 72 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 53

TABLE 4.1
Characteristics of Wastewater Streams in MWTP
COD TSS VSS NH3-N TN TP
(mg L ) (g L ) (g L ) (mg L ) (mg L ) (mg L )
−1
−1
−1
−1
−1
−1
Centrate 3027 ± 779 0.46 ± 0.28 0.35 ± 0.26 113 ± 18 150 250
Municipal 224.0 ± 4.2 – – – 38.95 ± 1.91 6.86 ± 0.05
wastewater
COD: Chemical Oxygen Demand; TSS: Total Suspended Solid; TN: Total Nitrogen; TP: Total
Phosphorous.
Fortunately, the carbon, nitrogen, and phosphorus composition ratios in centrate
are favorable for growing algae. This makes centrate attractive as a nutrient source
for the production of microalgae, which can be converted into biofuels for vehicle
use (Wang et al., 2010). Studies have shown that algae grown with centrate usually
have higher yields, higher algae growing density, and fewer inputs than other waste-
water streams such as raw municipal wastewater and manure (Mu et al., 2014). One
study shows that centrate can be directly added into algae growing reactors without
dilution (Min et al., 2011). The pollutants in the effluent of algae cultivation can be
reduced significantly, which will reduce the load in the aeration tank.
Besides centrate, other liquid waste is created, such as supernatant of sludge
thickening and sludge digestion. The liquid waste contains high levels of COD, N,
and P as well. Instead of being discharged from the plant, these other liquid streams
are usually recycled to the activated sludge process, which raises the treating load of
the aeration tank. Injecting those streams into algae cultivation can reduce the pol-
lutants in those steams and co-produce biofuels for the plant’s use.

4.3 TECHNOLOGIES FOR RENEWABLE ENERGY FROM WASTE
CREATED IN WASTEWATER TREATMENT FACILITIES
4.3.1 anaerobic DigesTion of biosoliDs
Anaerobic digestion is a commonly used method to convert biosolids into biogas, which
generates electricity and heat. It is a natural biochemical process in which microorgan-
isms break down and digest the solid biomass in anaerobic conditions. As the micro-
organisms are breaking down the biosolids, they create biogas— usually comprised of
methane (CH ) and carbon dioxide (CO )—and solid and liquid residues called diges-
2
4
tate. A liquid layer, supernatant, is also created on the top of the digestate and discharged
out of the digester. In a municipal wastewater treatment plant, digestion is usually done
in mesophilic conditions, where the biosolids are in tanks at ~30–38°C to increase yield
and eliminate harmful pathogens, because more methane can be generated.
Biogas is typically made up of approximately 50%–80% methane and approxi-
mately 20%–50% carbon dioxide. In the Greenhouse Gases, Regulated Emissions,
and Energy Use in Transportation (GREET) database, methane generation rate is