Organic Waste Recycling

238 Organic waste recycling: technology and management

The AFPs are designed to retain the settleable solids in wastewater and to
foster their long-term fermentation to gaseous products such as methane, carbon
dioxide, and ammonia. The HRAPs are designed to produce algae and
photosynthetic oxygen in sufficient, but not excessive, amounts to permit
microbial oxidation of organic residuals. Algae in the HRAPs tend to raise the
water pH and a pH of 9.2 for 24 hours will provide an almost 100% kill of E.
coli and presumably most pathogenic bacteria (Oswald 1991). It is not
uncommon for HRAPs to reach pH levels of 9.5 or 10 during the day, so they
have a high disinfection rate. The ASPs are designed to settle and remove algal-
bacterial concentrates from the oxidized waste; and the AMPs are designed to
hold the treated wastewater for reuse in aquaculture or seasonal applications of
irrigation water for controlled discharges and further improve the disinfection.
Table 5.7 shows the performance of AIWPS. The algal biomass production
ranges from 80 to 220 kg (dry weight) / (ha-day).

Table 5.7 Performance of AIWPS (Oswald 1991)

(a) St Helena, California
Parameter Influent
sewage

AFP HRAP ASP AMP %

removal

HRT (days)

BOD 5 (mg/L)

COD (mg/L)

Total C (mg/L)

Total N (mg/L)

Total P (mg/L)

0

220

440

210

40

14

20

20

120

140

16

13

10

9

70

90

13

12

5

6

60

70

6

8

• 7
  30
  50
  4
  5

97

93

77

90

64

(b) Hollister, California
Parameter Influent
sewage

AFP HRAP ASP AMPa %

removal

HRT (days)

BOD (mg/L)

TVS (mg/L)

E. coli (MPN/ 100 mL)

0

194

604

108

32

43

393

106

10

7

341

105

7

7

347

104

96
42
99.999
a Hollister's settling pond effluent is discharged to natural gravel percolation beds; there

is no surface effluent

5.2.2 Estimation of algal production

A detailed literature review of photosynthetic algal yield was conducted by
Goldman (1979b). He summarized that at best conversion efficiencies of less
than 5% of total sunlight into algal biomass can be expected, leading to the
upper limit in yields of 30-40 g (dry weight)/ (m^2 -day). Although high
temperature leads to high algal yields, respiratory and other decay processes are