Organic Waste Recycling

226 Organic waste recycling: technology and management

System (b) is related with our objectives of waste treatment, recovery and
recycling and will be the main concern in this chapter.
The basic reactions occurring in an algal pond can be represented by
Equations 2.1, 2.2 and 2.6 or the “algal-bacterial symbiosis” previously cited in
section 2.4. These reactions are schematically shown in Figure 5.4. Organic
matter entering the system as wastewater or sludge is aerobically decomposed
by bacteria, using oxygen produced by algal photosynthesis. The algae, utilizing
solar energy and nutrients (or by-products) from the bacterial oxidation, perform
photosynthesis and synthesizing new algal biomass. It is apparent from Figure
5.4 that the excess biomass of algae and bacteria produced during the algal-
bacterial symbiosis needs to be regularly removed from the system to maintain a
constant biomass and efficient performance of the system.

Figure 5.4 The cycle of oxygen and algal production in sewage treatment by photo-
synthesis (from Oswald and Gotaas 1955; reproduced by permission of the American
Society of Civil Engineers)

5.2.1 High-rate algal pond (HRAP) systems

The HRAP conventionally takes the form of a continuous channel equipped
with an aerator-mixer to re-circulate the contents of the pond. It is characterized
by large area/volume ratios, and shallow depths in the range of 0.2-0.6 m to
allow sunlight to penetrate the whole pond depth. To minimize short-circuiting,
baffles are normally installed in the pond to make the length/width ratio of the
channel greater than 2/1. A diagram and photograph of HRAP are shown in

DISSOLVED

OXYGEN

EXCESS

ALGAE

SOLAR

ENGERY

ORGANIC

WASTE

EXCESS

BACTERIA

BACTERIAL

OXIDATION

ALGAL

PHOTOSYNTHESIS

CO 2 + H 2 O + NH 4 +

CHOROPHYLL