Chapter 24
Wastewater Treatment and Biosolids Reuse
Charles P. Gerba and Ian L. Pepper
1. What are the three major steps in modern wastewater treatment?
Primary, secondary and tertiary treatment.
2. Why is it important to reduce the amount of biodegradable organic matter and
nutrients during sewage treatment?
Because discharge of sewage into rivers and other natural bodies of water would
result in anaerobic conditions killing fish and other organisms dependent upon
oxygen in the water.
3. When would tertiary treatment of wastewater be necessary?
Tertiary treatment of wastewater is practiced for additional protection of
wildlife after discharge into rivers and lakes. It is also performed when the
wastewater is to be reused for irrigation, recreational purposes, or for drinking water.
4. What are some types of tertiary treatment?
Most processes involve some type of physicochemical treatment, such as
coagulation, filtration, activated carbon adsorption of organics, reverse osmosis, and
additional disinfection. Land application and wetlands may also be considered tertiary
forms of treatment.
5. What are the processes involved in the removal of heavy metals from wastewater
during treatment by artificial wetlands?
Adsorption to plant and debris surfaces.
6. What are the three types of land application of wastewater? Which one is most
likely to contaminate the groundwater with enteric viruses? Why? What factors
determine how far viruses will be transported in groundwater? How does nitrogen
removal occur? How does phosphorus removal occur?
Low-rate irrigation, overland flow and high-rate infiltration. High-rate infiltration,
because of the use of sandy coarse soils. Virus removal is influenced by infiltration
rates, electrostatic charge on the virus, hydrophobic interactions, van der Waals
forces, type of soil, water pH salts in solution, types of virus and soluble organic
matter (see Chapter 19). Nitrogen may be taken up by the plants or by denitrification
by the soil bacteria. Phosphorous is immobilized within the soil matrix by fixation or
precipitation.
7. What are the major contaminates in groundwater associated with the use of onsite treatment systems?
Nitrates, enteric viruses.
8. What factors may determine the concentration of enteric pathogens in domestic
raw sewage?
The incidence of infection within the community, the socioeconomic status of the
population, the time of year, the per capita water consumption.
9. Five milliliters of a wastewater sample is added to dilution water in a 300-ml
BOD bottle. If the following results are obtained, what is the BOD after 3 days
and 5 days?
Using Eq. 24.1
Thus:
BOD3(mg/L) = D1 – D3 / P
D1 = 9.55 D3 = 3.20
D5 = 2.40
P = 5 mL/300 mL = 1.66% = 0.0166
Thus BOD3 = 9.55 – 3.20/ 0.0166 = 6.35/0.0166 = 382.5 mg/L
BOD5 = 9.55 – 2.40/0.0166 = 7.15/0.0166 = 430.7 mg/L
10. What is the difference between Class A and Class B biosolids? Name three
processes that can be used to produce Class A biosolids.
Class A biosolids are treated to reduce the presence of pathogens to below detectable
levels and can be used without any pathogen-related restrictions at the application
site. Class B biosolids are also treated to reduce pathogens but still contain detectable
levels of them. Also see Information Box 24.4. Examples of Class A processes are
composting, lime treatment, heat treatment, and thermophilic digestion.
11. List some advantages and disadvantages of the wetland treatment of sewage.
They can reduce the levels of biodegradable organics, suspended solids, nitrogen,
phosphorus, heavy metals, trace organics and pathogens (see Table 24.9). They
require a large land area and when used for production of aquatic animals for human
consumption there may be a health risk.
12. What is the major mechanism of pathogen removal during activated sludge
treatment?
Adsorption of the viruses to the sludge floc is the major mechanism of removal.
13. What treatment processes would you need to obtain an 8-log reduction of enteric
viruses? Giardia?
The reductions that can be achieved by different wastewater treatment processes are
listed in Table 24.6 and Table 24.5.
14. How effective do you think sunlight is in killing Cryptosporidium oocysts?
Enteric viruses?
Cryptosporidium is very sensitive to the UV light found in sunlight and sunlight can
cause a reduction in the infectivity of the organism (see Table 24.9 and Table 26.7).
Enteric viruses are more resistant to the UV light in sunlight (see Table 24.9 and
Table 26.7).