Transcript Microbial Reductions in Wastewater Treatment

Previously
• Microbial survival in environmental media
– Water, land, air
• Small scale sanitation, composting latrines, etc
• Conventional wastewater treatment in developed
countries
– Screening/settling/”activated sludge”/disinfection
– Overview of processes, chemical and physical
Outline
• But what about developing countries?
– In many cases, no wastewater treatment
– “Natural attenuation”
• What are some practical options for
wastewater treatment in poor countries?
– (& see handouts)
• More about microbes and how they are
reduced in wastewater
Alternative Biological Treatment of Wastewater:
Alternatives for Small and Rural Communities
• Letting nature do its thing
• Lagoons, Ponds and Ditches
– aerobic, anaerobic and facultative; for smaller
communities and farms
– enteric microbes are reduced by ~90-99% per pond
• multiple ponds in series increases microbe reductions
• Constructed Wetlands
– aerobic systems containing biologically active, oxidizing
microbes and emergent aquatic plants
• Lagoons and constructed wetlands are practical and
economical sewage treatment alternatives when land is
available at reasonable cost (Phnom Penh)
Facultative Oxidation (Waste Stabilization)
Pond
Stabilization Ponds or Lagoons
• Aerobic and Facultative Ponds:
• Biologically Rx by complementary activity of algae and
bacteria.
• Used for raw sewage as well as primary- or
secondary-Rx’d. effluent.
• Bacteria and other heterotrophs convert organic matter
to carbon dioxide, inorganic nutrients, water and
microbial biomass.
• Algae use CO2 and inorganic nutrients, primarily N and
P, in photosynthesis to produce oxygen and algal
biomass.
• Many different pond designs have been used to treat
sewage
• Used here in the US in decentralized plants and CAFOs
Stabilization Ponds or Lagoons, cont
• Facultative ponds: upper, aerobic zone and a
lower anaerobic zone.
• Aerobic heterotrophics and algae proliferate in
the upper zone.
• Biomass from upper zone settles into the
anaerobic, bottom zone.
• Bottom solids digested by anaerobic bacteria
• Also used with aquaculture systems
Enteric Microbe/Pathogen Reductions in
Stabilization Ponds
• BOD and enteric microbe/pathogen reductions of 90%,
esp. in warm, sunny climates.
• Even greater enteric microbe /pathogen reductions by
using two or more ponds in series
• Better BOD and enteric microbe/pathogen reductions if
detention (residence) times are sufficiently long (several
weeks to months)
• Enteric microbes reduced by 90% in single ponds and by
multiples of 90% for ponds in series.
• Microbe removal may be quite variable depending upon
pond design, operating conditions and climate.
– Reduction efficiency lower in colder weather and shorter
retention times
Constructed Wetlands and Enteric Microbe
Reductions
• Surface flow (SF) wetlands reduce enteric microbes
by ~90%
• Subsurface flow (SSF) wetlands reduce enteric
microbes by ~99%
• Greater reduction in SSF may be due to greater
biological activity in wetland bed media (porous
gravel) and longer retention times
• Multiple wetlands in series incrementally increase
microbial reductions, with 90-99% reduction per
wetland cell.
Septic Tank-Soil Absorption Systems for On-Site
Sewage Rx
• Used where there are no sewers and community sewage
treatment facilities: ex.: rural homes
• Septic tank: solids settle and are digested
• Septic tank effluent (STE) is similar to primary sewage
effluent
• Distribute STE to soil via a sub-surface, porous pipe in a
trench
• Absorption System: Distribution lines and drainfield
• Septic tank effluent flows through perforated pipes located
2-3 feet below the land surface in a trenches filled with
gravel, preferably in the unsaturated (vadose) zone.
– Effluent discharges from perforated pipes into trench gravel
and then into unsaturated soil, where it is biologically treated
aerobically.
• Enteric microbes are removed and retained by the soil
More on sanitation technologies:
• www.wateraid.co.uk
• Handouts
• For more on small-scale sanitation, check out
The Humanure Handbook
Log10 Reduction of Pathogens by Wastewater Rx
Processes
Log10 Reduction of Pathogens by Wastewater Rx
Processes
Indicator Microbe Levels in Raw and Treated Municipal
Sewage: Sewage Treatment Efficacy
Number/100 ml
100000000
10000000
1000000
100000
10000
1000
100
10
1
F. col.
E. coli
Ent.
C. p. F+ phg.
Raw
Treated
(geom. mean values of 24 biweekly samples)
REMOVAL OF ENTERIC BACTERIA BY
SEWAGE TREATMENT PROCESSES
ORGANISM
Fecal indicators
E. coli
PROCESS
Primary sed.
Primary sed.
Fecal indicators
Fecal indicators
Fecal indicators
temp.
Trickling filt.
20-80%
Activated sludge
40-95%
Stab. ponds, 1 mo. >99.9999% @ high
Salmonellae
Salmonellae
Primary sed.
"
% REMOVAL
0-60%
32-50%
79%, 6-7 hrs.
73%, 6-7 hrs.
Entamoeba histolytica Reduction by Sewage
Treatment
ORGANISM
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
PROCESS
Primary Sed.
Primary Sed., 2 hr.
Primary sed., 1 hr.
Primary sed. + Trickl. Filt.
Primary sed. + Act. Sludge
% REMOVAL
50%
64%
27%
25%
83%
E. histolytica
E. histolytica
Oxidation ditch + Sedimentation
91%
Stabilization ponds + sedimentation 100%
E. histolytica
E. histolytica
E. histolytica
"
100%, 94%, 87%
"
100%
Aerated lagoon (no settling)
84%
Microbial Reductions by Wastewater Treatment
% Reduction
Microbe 1o&2o Filt. Disinfect. Store Total Rx.
Tot. colif.
98
69
99.99
75
99.99999
Fec. colif.
99
10
99.998
57
99.999996
Coliphage
82
99.98
90
90
99.99997
Enterovirus
Giardia
98
84
96
91
99.999
93
99
78
50
99.9993
Cryptosporidium
93
98
61
<10
99.95
Disinfection of Wastewater
• Intended to reduce microbes in 1o or 2o treated effluent
– Typically chlorination
– Alternatives: UV radiation, ozone and chlorine dioxide
• Good enteric bacterial reductions: typically, 99.99+%
– Meet fecal coliform limits for effluent dicharge
• Often 200-1,000 per 100 ml geometric mean as permitted
discharge limit
• Less effective for viruses and parasites: typically, 90-99%
reduction
• Toxicity of chlorine and its by-products to aquatic life now
limits wastewater chlorination; may have to:
– Dechlorinate
– Use an alternative, less toxic chemical disinfectant or
– Use an alternative treatment process to reduce enteric
microbes
When Wastewater Disinfection is
Recommended or Required
• Discharge to surface waters:
– near water supply intakes
– used for primary contact recreation
– used for shellfish harvesting
– used for irrigation of crops and greenspace
– other direct and indirect reuse and reclamation purposes
• Discharge to ground waters waters:
– used as a water supply source
– used for irrigation of crops and greenspace
– other direct and indirect reuse and reclamation purposes
Wastewater Reuse: reading for Thursday
• Wastewater is sometimes reused for beneficial, non-potable
purposes in arid and other water-short regions.
• Often uses advanced or additional treatment processes,
sometimes referred to as “reclamation”
• Biological treatment in “polishing” ponds and constructed
wetlands
• Physical-chemical treatment processes as used for drinking
water:
– Coagulation-flocculation and sedimentation
– Filtration: granular medium filters; membrane filters
– Granular Activated Carbon adsorption
– Disinfection