General Statement of Activated Sludge Processes
A typical wastewater treatment plant is shown schemati cally in Figure 2. It usually consists of four sections, namely pretreatment, primary treatment, secondary treatment, and tertiary treatment. There are no clear classification criteria to distinguish between pretreatment
|
Parameter |
Primary sludge |
Secondary sludge |
|
Total solids (TS) (%) |
3.0-7.0 |
0.5-2.0 |
|
Volatile solids (% of TS) |
60-80 |
50-60 |
|
Nitrogen (N, % ofTS) |
1.5-4.0 |
2.4-5.0 |
|
Phosphorous (P2O5, % of TS) |
0.8-2.8 |
0.5-0.7 |
|
Potash (K2O, % of TS) |
0-1.0 |
0.5-0.7 |
|
Heat value (kJ kg 1, dry basis) |
23000-30000 |
18500-23000 |
|
pH |
5.0-8.0 |
6.5-8.0 |
|
Alkalinity (mg l 1 as CaCO3) |
500-1500 |
580-1100 |
Data from Metcalf E (1991) Wastewater Engineering. New York: McGraw Hill; Chen G, Yue PL, and Mujumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883 916.
Data from Metcalf E (1991) Wastewater Engineering. New York: McGraw Hill; Chen G, Yue PL, and Mujumdar AS (2002) Sludge dewatering and drying. Drying Technology 20: 883 916.
Table 3 Sludge indices from settleability tests
Property
Representation
Experimental condition
Sludge volume index (SVI)
Sludge density index (SDI) Fractional sludge depth (fsd)
Stirred specific volume index (SSVI)
Dilute sludge volume index (DSVI)
V settled volume of sludge Vt total volume
SDI 100/SVI
depth of sludge layer fsd 1
SSVI
DSVI
depth of column fsd x 1000 MLSS
MLSS V <200 ml n twofold dilutions required
30 min quiescent settlement
As above
30 min settlement under gently stirred conditions (1-2 rpm stirring mechanism)
Dilution at which the settled sludge volume (V) is less than, but closest to 200 ml
Table 4 Types of sewage sludge
Type of sewage sludge
Characteristics
South Africa classification
Raw sludge Primary sludge Secondary sludge
Aerobic digested sludge Anaerobic digested sludge
Chemically conditioned sludge
Heat treated sludge Composted sludge
Unstable, causing odor nuisance and fly-breeding Contains pathogenic organisms
Fully or partly stabilized without significant odor nuisance and fly-breeding Contains pathogenic organisms
Certified to comply certain quality requirements
Type A
Type B
Types C and D (if not certified, type B)
Table 5 Biological groups in activated sludge
Biological group Genus (example) Species (example) Function
Autotrophic microbes Nitrosomonas N. europaea Nitrogen removal
Heterotrophic microbes Thiosphaera pantotropha Simultaneous nitrification and denitrification
Pseudomonas Nitrification and phosphorous removal
Cyanobacteria Anabaena sphaerica Carbon and nitrogen removal
Acinetobacter Gram-negative and others Phosphorous removal Protozoa Secondary colonization
- Figure 1 Structure of Anabaena sphaerica.
and primary treatment in the literature. For simplicity, we combine both sections together in this article. Since the tertiary treatment only involves a small amount of sludge, it is not described in detail.
A photo of the wastewater treatment plant of Buelach, Switzerland is shown in Figure 3, which consists of rectangular primary settlers, octagonal, aeration basins, circular secondary settlers, and anaerobic reaction tanks.
The unit operations in pretreatment and primary treatment, as well as in secondary treatment are explained in the following section.
Post a comment