REHS Exam: Private Onsite Wastewater
Treatment and Disposal UPDATED
QUESTIONS AND CORRECT ANSWERS
Biochemical Oxygen Demand (BOD) - CORRECT ANSWER The difference between the
initial dissolved O2 in a water sample and dissolved O2 in a duplicate sample after a stated period of
time (usually 5 days). Best single measure of wastewater or polluted water.
Chemical Oxygen Demand (COD) - CORRECT ANSWER A measure of the amount of O2
chemically consumed by the oxidation of organic and oxidizable inorganic materials in a water
sample. Good for measuring industrial wastes.
Domestic Sewage - CORRECT ANSWER used water from a home or community. Includes
toilet, bath, laundry, lavatory, and kitchen-sink wastes. Sewage strength is reported in BOD, SS, and
COD
National Pollutant Discharge Elimination System (NPDES) - CORRECT ANSWER System
for issuance of permits for the discharge of treated sanitary, industrial, and commercial wastes under
the 1972 Federal Water Pollution Control Act. Permit specifies the treatment and expected outcome of
effluent to protect water quality.
Privy - CORRECT ANSWER where excreta is disposed without the aid of water; odor
prevention is via 4" vent extending 18" above roof peak.
Point Source Pollution - CORRECT ANSWER any discernible, confined, or discrete
conveyance from which pollutants are or my be discharged
Nonpoint Pollution - CORRECT ANSWER not a point source and is manmade/man-induced
resulting in the alteration of the chemical, physical, biological or radiological integrity of water.
How a septic tank works - CORRECT ANSWER purpose: 24-72 hour watertight detention
tank, condition sewage to be more readily percolated into the subsoil
functions: removal of solids, biological treatment, sludge and scum storage
, Sewage Treatment Process - CORRECT ANSWER Anaerobic digestion: operates in anoxic
environment, treats sewage with anaerobic bacteria, very slow process (e.g. septic tank)
Aerobic digestion: operates in an aerobic environment, treats sewage with aerobic bacteria, rapid
treatment process, produces lowest suspended solid effluent (e.g. aerobic treatment plant)
Design Flow - CORRECT ANSWER Residential: usually based on number of bedrooms,
sometimes based on water fixtures
Non-residential: usually based on code table for establishment type, better to use actual metered water
usage figures
perc test - CORRECT ANSWER Test to determine drainage capability of soil. Calculate how
many minutes it takes the water to drop one inch. Perform at least three tests for the average lot (6
tests preferred), small subdivision three test holes per acre.
Permitted Percolation Rates - CORRECT ANSWER Rates under 1 minute are unsuitable soils
Rates under 5 minutes are dependent on distance to well, bodies of water, depth to limiting layer,
BOD, TSS and more.
Use 6 to 15 minutes as a guide.
Soil Characteristics - CORRECT ANSWER gravel (greater than 2mm)
sand (2mm to 0.05mm)
Silt (0.25mm to 0.002mm)
Clay (less than 0.002mm)
Loam soil contains sand, silt, and clay in many different proportions (ideal % = 40/40/20)
soil permeability - CORRECT ANSWER Sandy soil with some gravel and well drained is
suitable for drainage field
Clay loam and clay do not drain well and are usually considered unsuitable
procedure involves evaluating a soil core or test pit
Procedure yields and permeability estimate
Treatment and Disposal UPDATED
QUESTIONS AND CORRECT ANSWERS
Biochemical Oxygen Demand (BOD) - CORRECT ANSWER The difference between the
initial dissolved O2 in a water sample and dissolved O2 in a duplicate sample after a stated period of
time (usually 5 days). Best single measure of wastewater or polluted water.
Chemical Oxygen Demand (COD) - CORRECT ANSWER A measure of the amount of O2
chemically consumed by the oxidation of organic and oxidizable inorganic materials in a water
sample. Good for measuring industrial wastes.
Domestic Sewage - CORRECT ANSWER used water from a home or community. Includes
toilet, bath, laundry, lavatory, and kitchen-sink wastes. Sewage strength is reported in BOD, SS, and
COD
National Pollutant Discharge Elimination System (NPDES) - CORRECT ANSWER System
for issuance of permits for the discharge of treated sanitary, industrial, and commercial wastes under
the 1972 Federal Water Pollution Control Act. Permit specifies the treatment and expected outcome of
effluent to protect water quality.
Privy - CORRECT ANSWER where excreta is disposed without the aid of water; odor
prevention is via 4" vent extending 18" above roof peak.
Point Source Pollution - CORRECT ANSWER any discernible, confined, or discrete
conveyance from which pollutants are or my be discharged
Nonpoint Pollution - CORRECT ANSWER not a point source and is manmade/man-induced
resulting in the alteration of the chemical, physical, biological or radiological integrity of water.
How a septic tank works - CORRECT ANSWER purpose: 24-72 hour watertight detention
tank, condition sewage to be more readily percolated into the subsoil
functions: removal of solids, biological treatment, sludge and scum storage
, Sewage Treatment Process - CORRECT ANSWER Anaerobic digestion: operates in anoxic
environment, treats sewage with anaerobic bacteria, very slow process (e.g. septic tank)
Aerobic digestion: operates in an aerobic environment, treats sewage with aerobic bacteria, rapid
treatment process, produces lowest suspended solid effluent (e.g. aerobic treatment plant)
Design Flow - CORRECT ANSWER Residential: usually based on number of bedrooms,
sometimes based on water fixtures
Non-residential: usually based on code table for establishment type, better to use actual metered water
usage figures
perc test - CORRECT ANSWER Test to determine drainage capability of soil. Calculate how
many minutes it takes the water to drop one inch. Perform at least three tests for the average lot (6
tests preferred), small subdivision three test holes per acre.
Permitted Percolation Rates - CORRECT ANSWER Rates under 1 minute are unsuitable soils
Rates under 5 minutes are dependent on distance to well, bodies of water, depth to limiting layer,
BOD, TSS and more.
Use 6 to 15 minutes as a guide.
Soil Characteristics - CORRECT ANSWER gravel (greater than 2mm)
sand (2mm to 0.05mm)
Silt (0.25mm to 0.002mm)
Clay (less than 0.002mm)
Loam soil contains sand, silt, and clay in many different proportions (ideal % = 40/40/20)
soil permeability - CORRECT ANSWER Sandy soil with some gravel and well drained is
suitable for drainage field
Clay loam and clay do not drain well and are usually considered unsuitable
procedure involves evaluating a soil core or test pit
Procedure yields and permeability estimate
