TABLE 6. Typical Efficiencies of Sewage-Treatment Methods*
Percentage reduction
Suspended
Treatment matter BOD Bacteria
Fine screens 5-20 ... 10-20
Plain sedimentation 35-65 25-40 50-60
Chemical precipitation 75-90 60-85 70-90
Low-rate trickling filter, with pre-
and final sedimentation 70-90+ 75-90 90+
High-rate trickling filter with pre-
and final sedimentation 70-90 65-95 70-95
Conventional activated sludge with
pre- and final sedimentation 80-95 80-95 90-95+
High-rate activated sludge with
pre- and final sedimentation 70-90 70-95 80-95
Contact aeration with pre- and
final sedimentation 80-95 80-95 90-95+
Intermittent sand filtration with
presedimentation 90-95 85-95 95+
Chlorination:
Settled sewage ... t 90-95
Biologically treated sewage ... t 98-99
* Steel—Water Supply and Sewerage, McGraw-Hill.
!Reduction is dependent on dosage.oxygen demand is usually about 0.5 Ib/day (0.23 kg/day). To convert the strength of an
industrial waste to the same base used for sanitary waste, apply the population equivalent
relation in step 5. Some cities use the population equivalent as a means of evaluating the
load placed on the sewage-treatment works by industrial plants.
Table 7 shows the products resulting from various sewage-treatment processes per
million gallons of sewage treated. The tabulated data are useful for computing the volume
of product each process produces.
Environmental considerations are leading to the adoption of biogas methods to handle
the organic fraction of municipal solid waste (MSW). Burning methane-rich biogas can
meet up to 60 percent of the operating cost of waste-to-energy plants. Further, generating
biogas avoids the high cost of disposing of this odorous by-product. A further advantage
is that biogas plants are exempt from energy or carbon taxes. Newer plants also handle in-
dustrial wastes, converting them to biogas.
Biogas plants are popular in Europe. The first anaerobic digestion plant capable of
treating unsorted MSW handles 55,000 mt/yr. It treats wastestreams with solids contents
of 30 to 35 percent. Automated sorting first removes metals, plastics, paperboard, glass,
and inerts from the MSW stream. The remaining organic fraction is mixed with recycled
water from a preceding compost-drying press to form a 30 to 35 percent solids sludge
which is pumped into one of the plant's three 2400-m^3 (84,720 ft^3 ) disgesters.
Residence time in the digester is about 3 weeks with a biogas yield of 99 m^3 /mt of
MSW (3495 ft^3 /t), or 146 mVmt (5154 ft^3 /t) of sorted organic fraction. Overflow liquid
from the digester is pressed, graded, and sold as compost. Mixtures of MSW, sewage