Green Chemistry and the Ten Commandments

Air

Sludge

settling

Purified    water

with    reduced

BOD

Wastewater  containing

BOD,    {CH 2 O}

Aeration    tank

Degradation of  BOD

Mineralization  of  C,  N,  P,  and S

Settled sludge  with    viable  microorganisms

Excess  sludge  to

anaerobic   digester

Anaerobic   digester

Byproduct   methane

fuel    gas

2{CH 2 O} → CH 4    +   CO 2

Figure 7.10. Activated sludge process.

In addition to converting organic C to CO 2 , the bacteria in the aeration tank convert
organic N, P, and S to simple inorganic ions, such as NH 4 +, H 2 PO 4 - , and SO 42 - , a process
called mineralization. After an appropriate residence time in the tank, the treated water is
taken to a settling tank where the microorganisms settle out as sludge. Most of this sludge
is pumped back to the front of the aeration tank to contact additional BOD. Some excess
sludge is taken to an anaerobic digestor, where the sludge is acted upon by methane-
forming bacteria in the absence of oxygen to generate methane gas, CH 4. The purified
water from the treatment plant can be discharged directly or subjected to additional
treatment. One option is to allow the water to flow slowly through a constructed wetland
system where it is purified by natural processes.
Sewage treatment has a significant potential for the practice of green chemistry.
One of the most obvious ways in which this can occur is by efficient utilization of the
methane produced in the anaerobic digestion of the spent sludge from the activated
sludge process. Processes that rely primarily on anaerobic digestion have the potential to
produce even more methane. Such anaerobic processes are used in some large livestock
feeding operations, providing significant amounts of useful methane. Even after anaerobic
digestion, the leftover sludge has value as fertilizer to provide nitrogen and phosphorus

190 Green Chemistry, 2nd ed