Transcript INTRODUCTION TO MYCOLOGY

MIC 303
INDUSTRIAL AND
ENVIRONMENTAL
MICROBIOLOGY
CHAPTER 10-SEWAGE
(WASTEWATER) TREATMENT
Composition of Domestic Wastewater
• Combination of human and animal excreta
(feces and urine) and gray water resulting from
washing, bathing and cooking.
• Mainly composed of:
 Proteins (40-60%)
 Carbohydrates (25-50%)
 Fats and oils (10%)
 Urea derived from urine
 Trace organic compound (inc pesticides,
surfactants, phenols and pollutants such as
non metal (As, Se), metals (Cd, Hg, Pb),
benzene
compounds,
chlorinated
compounds.
Overview of Wastewater Treatment
• Treatment methods based on chemical and
biological processes called unit processes.
↓
• Chemical unit processes include disinfection,
adsorption or precipitation.
• Biological unit processes involve microbial
activity,
responsible
for
organic
matter
degradation and removal of nutrients.
Wastewater Treatment Processes
• Wastewater treatment comprises four steps:
1) Preliminary treatment: to remove debris and coarse
materials that may clog equipment in the plant.
2) Primary treatment: treatment by physical processes such as
screening and sedimentation.
3) Secondary treatment: Nutrient removal also generally
occurs during secondary treatment of wastewater.
 Biological (eg: activated sludge, trickling filter, oxidation
ponds)
 Chemical (e.g. disinfection)
4) Tertiary or advanced treatment: Biological and chemical
unit processes used to further remove BOD, nutrients,
pathogens and parasites and toxic substances.
Wastewater Treatment Processes (Con’t)
• Primary treatment
 Removal of solids
 Disinfection
• Secondary treatment
 Removal of much of the BOD
 Disinfection
 Water can be used for irrigation
• Tertiary treatment
 Removal of remaining BOD, N, and P
 Disinfection
 Water is drinkable
Municipal Sewage Treatment
PRIMARY TREATMENT-Septic Tanks
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•
Often use to discard domestic human waste for
homes and business in areas of low population
density that are not connected to municipal
sewage systems.
Septic tanks: a device whose operation is similar
in principle to primary treatment.
Sewage enters a holding tank and suspended
solids settle out.
The sludge in the tank must be pumped out
periodically and disposed off.
The effluent flows through a system of perforated
piping into a leaching (soil drainage) field.
The effluent enter the soil → decomposed by soil
microorganisms.
Septic Tanks (Con’t)
Limitations:
• The system work well when not
overloaded and drainage system is
properly sized to the load and soil type.
• For heavy clay soils, require extensive
drainage
systems
→
soil’s
poor
permeability.
• High porosity of sandy soils can results in
chemical or bacterial pollution of nearby
water supplies.
Primary Treatment: Septic Tanks
Oxidation Ponds
• Use by small communities and industries.
• Sewage (livestock waste) is dump into ponds called
lagoons or stabilization ponds.
• Inexpensive but require large areas of land.
• Incorporate 2 stages:
1) Pond 1: Settle solids, pump water to pond 2
(analogous to primary treatment). The effluent will
be pumped into second pond.
2) Pond 2: Pond is aerated by wave action.
 Difficult to maintain aerobic condition for
bacterial growth. However, growth of algae
helps to use carbon dioxide, thus producing
oxygen for bacterial consumption.
Bacterial decomposition of dissolved organic
matter in water.
Oxidation Ponds
SECONDARY SEWAGE TREATMENT
• Predominantly biological:
1) Reduce most of remain organic matter
from primary treatment.
2) Reduce the BOD.
• Sewage undergoes strong aeration → to
encaurage the growth of aerobic bacteria
and other microorganisms that oxidize the
dissolved organic matter to carbon dioxide
and water.
• Two common methods:
1) Activated sludge systems
2) Trickling filters
Activated sludge systems
• Use aeration tanks: air or pure oxygen is passed through the
effluent from primary treatment.
• Inoculum (Effluent from primary treatment) also termed
“activated sludge”.
• Activated sludge contains large numbers of sewagemetabolizing microbes, esp species of Zoogloea bacteria.
• This microbial community form bacteria-containing masses in
the aeration tanks called floc (sludge granules)
• The soluble organic matter in the sewage is incorporated into
the floc and its microorganisms.
• Aeration is discontinued after 4 to 8 hours and the contents of
tank are transferred to a settling tank, the floc settles out →
removing much of the organic matter.
• The remove solids are treated in anaerobic sludge digester
(sludge didestion).
Activated sludge systems
• The clear effluent is disinfected and
discharged (Tertiary Treatment).
• Activated sludge systems quite efficient →
remove 75-95 % of the BOD from sewage.
• Limitations:
Phenomena of “bulking” (the sludge will
float rather than settle out.
The organic matter in the floc will flows out
with the discharge effluent → resulting in
local pollution.
Cause by filamentous bacteria (e.g
Sphaerolitus natans and Nocardia sp).
An Activated Sludge System
Trickling Filters
• The sewage is sprayed over a bed of rocks or
molded plastic.
• A biofilm of aerobic microbes grows on the rock
or plastic surfaces.
• Air circulates throughout the rock bed,
microorganisms attached in the bioflim will
oxidize organic matter trickling over the surfaces
into carbon dioxide and water.
• Less efficient than activated sludge system,
remove 80-85 % of BOD.
• Adv: less troblesome to operate and have fewer
problems from overloads or toxic sewage.
A Trickling Filter
A Trickling Filter
Sludge Characteristics
• Two classes of sludge:
• Class A sludge: contains no
detectable pathogens.
• Class B sludge: treated to reduce
numbers of pathogens below certain
levels.
• Can
be
used
as
fertilizers
(biofertilizer)
• Disadv:
potential
problem
is
contamination with heavy metals
that are toxic to plants.
Sludge Digestion
• The sludge resulted in secondary treatment often pumped to
anaerobic sludge digesters.
• Carried out in large tanks and oxygen is almost excluded.
• Anaerobic sludge digester designed to encourage the growth
of anaerobic bacteria (esp methane-producing bacteria) that
degrade organic solids to soluble substances and gases,
mostly methane (60-70 %) and CO2 (20-30 %).
• Methane used as a fuel for heating the digester and frequently
used to run power equipment in the plant.
• This system resulting in a large amounts of undigested sludge,
but it is stable and inert.
• This sludge is pumped to shallow drying beds or waterextracting filters.
• Then, it will used for landfill or as a soil conditioner (termed as
biosolids).
Anaerobic Sludge Digester
Figure 27.22
TERTIARY SEWAGE TREATMENT
• Secondary effluent contains
 Residual BOD
 50% of the original nitrogen
 70% of the original phosphorus
• Tertiary treatment relies on physical and chemical
methods, removes these by:
 Filtration through sand and activated charcoal →
remove small particulate matter and dissolved
chemicals.
 Chemical precipitation → Lime, alum and ferric
chloride precipitate phosphate compounds.
 Nitrogen is converted to ammonia and discharged into
the air in stripping towers.
The purified water is the clarified and safe for drinking.