Green Chemistry and the Ten Commandments

Waste   brine

Purified    water

Membrane    selectively

permeable   to  water

Pressurized

water

Figure 7.9. Dissolved impurities can be removed from water by forcing pure water for a membrane
selectively permeable to water in a process called reverse osmosis.

Sewage

The primary objective of sewage treatment is to remove oxygen-demanding
substances from wastewater. These are substances of mostly biological origin, abbreviated
{CH 2 O}, that undergo biodegradation and consumption of dissolved oxygen, thus
exerting a biochemical oxygen demand (BOD). Sewage treatment can be divided into the
three main categories of (1) primary treatment to remove grit, grease, and solid objects,
(2) secondary treatment to reduce BOD, and (3) tertiary treatment to further refine the
quality of the effluent water. This section addresses primarily secondary treatment.
Secondary wastewater treatment uses a mass of microorganisms in contact with
sewage and atmospheric oxygen to eliminate BOD by the reaction,

{CH 2 O} + O 2 → CO 2 + H 2 O (7.17.6)

a process that also generates additional biomass capable of degrading more BOD. The
microorganisms can be on a support that alternately contacts liquid sewage and air.
One system that uses that approach is the trickling filter in which sewage is sprayed
over rocks coated with microorganisms. However, the most commonly used process at
present is the activated sludge process shown in Figure 7.10. In this process wastewater
is pumped into one end of a large aeration tank through which air is bubbled. Viable
microorganisms suspended in the tank metabolize the degradable materials composing
the BOD in the sewage, converting some of the carbon to carbon dioxide and synthesizing
additional microorganisms by an overall reaction that can be represented by the following
equation:

{CH 2 O} + O 2 → CO 2 + H 2 O + Biomass (7.17.7)

Chap. 7. Water, The Ultimate Green Solvent 189