Environmental Engineering Reference
In-Depth Information
28.5 Energy Recovery from Wastewater
Effective utilization of the energy and nutrients removed from wastewater during the
treatment process is becoming more important as the need for raw materials and energy
costs increase. Wastewater treatment plant biosolids have attractive energy content; the
caloriic value of the dry sludge is almost equal to that of brown coal [50]. In the United
States, this means that the energy content in wastewater is approximately 2% of the annual
electricity demand. Nitrogen and phosphorus are both valuable nutrients used in fertil-
izers but harmful if released into the environment untreated, as they lead to overgrowth
in aquatic systems. The demand for nitrogen for fertilizer is on the order of 121 million
ton per year for synthetic fertilizers made with the Haber-Bosch process, which utilizes
the almost unlimited atmospheric nitrogen supply [51]. Phosphorus, however, is derived
from mineral phosphate rock, the supply of which is projected to become exhausted in
the next 50-100 years [51]. Ammonia is a source of nitrogen and can be harvested from
wastewater. In a recent study [43], it was shown that in addition to anaerobic digestion
for methane production, the additional extraction of ammonia can add an energy beneit
when the system is integrated systematically with energy-eficient operation. For high
total ammonia nitrogen content [TAN] of 10,000 mg/l, it was shown that with energy
capture eficiency of 35% for the hydrostatic pressure and heat generated by the chemical
reactions as well as utilization of the nitrogen/hydrogen product stream as the stripping
gas, 26.3% more energy could be produced with the duel fuel capture (with subsequent
processing of ammonia to generate hydrogen) than with methane alone. In fact, for [TAN]
concentrations higher than approximately 2000 mg/l, the system received a positive ben-
eit from methane and hydrogen production over the methane-only system. By using
innovative modular system integration, it was shown [43] that it is also possible to harvest
ammonia from waste streams without need for further reining to fuel and obtain a valu-
able industrial chemical resource.
Sewage sludge can be incinerated, pyrolyzed, gasiied, or digested to produce fuel, or
a combination of those methods [44]. The USEPA estimates that annual sewage sludge
generation in the United States exceeds 8 million tons dry weight, with 41% of the sludge
land applied, 22% incinerated, 17% landilled, and the remaining either put through an
advanced treatment process or other alternative beneicial use [52]. The USEPA has identi-
ied combined heat and power (CHP) applications involving anaerobic digesters for biogas
production (mainly methane) as a reliable and beneicial addition to many wastewater
treatment plants, providing the site with energy and heat independent of the electricity
grid and creating higher eficiencies by avoiding energy transmission and distribution. As
of 2011, CHP was feasible at 1351 wastewater treatment plants nationwide and represented
an economic payback in 7 years or fewer for between 252 and 662 of these sites and an
energy potential between 178 and 260 MW [53].
28.6 Energy Recovery from Water Purification Processes
In areas where excess pressure occurs in water distribution lines, which is often a result
of gravity as many systems are positioned to take advantage of available hydrostatic pres-
sure, pressure reduction valves are often used to ensure safe pressures at water taps.
