Biological treatment is an important process for wastewater treatment that utilizes microorganisms to break down organic impurities. The most common biological treatment process is activated sludge, which uses aerobic bacteria in an aeration tank to degrade wastewater. Trickling filters and oxidation ponds are alternative biological processes that are often used for smaller treatment systems. Trickling filters use a media like rock or plastic that bacteria grow on to treat wastewater as it trickles through. Oxidation ponds rely on algae and bacteria in large open ponds to break down organic matter through natural aerobic and anaerobic processes.

1. BIOLOGICAL TREATMENT

2.  Biological treatment is an important and integral part of any
wastewater treatment plant that treats wastewater from either
municipality or industry having soluble organic impurities or a mix of
the two types of wastewater sources.
 The obvious economic advantage, both in terms of capital investment
and operating costs, of biological treatment over other treatment
processes like chemical oxidation; thermal oxidation etc. has cemented
its place in any integrated wastewater treatment plant.

3. AEROBIC
 it is important to briefly discuss the term aerobic . Aerobic, as the title
suggests, means in the presence of air (oxygen).
 These term are directly related to the type of bacteria or
microorganisms that are involved in the degradation of organic
impurities in a given wastewater and the operating conditions of the
bioreactor.
 Therefore, aerobic treatment processes take place in the presence of
air and utilize those microorganisms (also called aerobes), which use
molecular/free oxygen to assimilate organic impurities i.e.
 convert them in to carbon dioxide, water and biomass. The anaerobic
treatment processes, on other hand take place in the absence of air (and
thus molecular/free oxygen) by those microorganisms (also called
anaerobes) which do not require air (molecular/free oxygen) to
assimilate organic impurities.

4. Aerobic Treatment Principle

6. ACTIVATED SLUDGE PROCESS (ASP)
SYSTEM
This is the most common and oldest biotreatment process
used to treat municipal and industrial wastewater.
Typically wastewater after primary treatment i.e. suspended
impurities removal is treated in an activated sludge process
based biological treatment system comprising aeration tank
followed by secondary clarifier.
 The aeration tank is a completely mixed or a plug flow (in
some cases) bioreactor where specific concentration of
biomass (measured as mixed liquor suspended solids (MLSS)
or mixed liquor volatile suspended solids (MLVSS)) is
maintained along with sufficient dissolved oxygen (DO)
concentration (typically 2 mg/l) to effect biodegradation of

7. The aeration tank is provided with fine bubble diffused
aeration pipework at the bottom to transfer required oxygen to
the biomass and also ensure completely mixed reactor.
Roots type air blower is used to supply air to the diffuser
pipework.
In several older installations, mechanical surface aerators have
been used to meet the aeration requirement.
The aerated mixed liquor from the aeration tank overflows by
gravity to the secondary clarifier unit to separate out the
biomass and allow clarified, treated water to the downstream
filtration system for finer removal of suspended solids.
 The separated biomass is returned to the aeration tank by
means of return activated sludge (RAS) pump. Excess biomass
(produced during the biodegradation process) is wasted to the

9. TRICKLING FILTERS
• Not a true filtering or sieving process
• Material only provides surface on which bacteria to grow
• Can use plastic media
• lighter - can get deeper beds (up to 12 m)
• reduced space requirement
• larger surface area for growth
• greater void ratios (better air flow)
• less prone to plugging by accumulating slime

10. TRICKLING FILTERS

11. Types of Trickling Filters
• Super rate
– synthetic plastic media units
• modules or random packed
• specific surface areas 2-5 times greater than rock
• much lighter than rocks
• can be stacked higher than rocks
– loading rates of 40-200 m3 wastewater/m2 filter
cross-sectional area-day
– plastic media depths of 5-10 m

12. • High rate
– single stage or two-stage rock media units
– loading rates of 10-40 m3 wastewater/m2 filter cross-
sectional area-day
– re-circulation ratio 1-3.
• Standard or low rate
– single stage rock media units
– loading rates of 1-4 m3 wastewater/m2 filter cross-sectional
area-day
– large area required

13. TRICKLING FILTERS
Filter Material

14. Flow Diagram for Trickling Filters
Recycle
Primary
clarifier
Trickling
filter
Final
clarifier
Waste
sludg
Influent

15. Typical Trickling Filter

16. Trickling Filter
• Tank is filled with solid media
– Rocks
– Plastic
• Bacteria grow on surface of media
• Wastewater is trickled over media, at top of tank
• As water trickles through media, bacteria degrade BOD
• Bacteria eventually die, fall off of media surface
• Filter is open to atmosphere, air flows naturally through media
• Treated water leaves bottom of tank, flows into secondary clarifier
• Bacterial cells settle, removed from clarifier as sludge
• Some water is recycled to the filter, to maintain moist conditions

17. Trickling Filter System

18. Trickling Filter Process
BACTERIA REMOVAL

19. Design Criteria for Trickling Filters
Table 10.5
Typical Design Criteria for Trickling Filters
Item Low-rate filter High-rate filter Super-rate filter
Hydraulic loading (m3
/m2
-d) 1 - 4 10 - 40 40 - 200
Organic loading (kg BOD5/m3-d) 0.08 - 0.32 0.32 - 1.0 0.8 - 6.0
Depth (m) 1.5 - 3.0 1.0 - 2.0 4.5 - 12.0
Recirculation ratio 0 1 - 3 1 - 4
Filter media Rock, slag, etc. Rock, slag,
synthetics
Filter flies Many Few, larvae are
washed away
Few or none
Sloughing Intermittent Continuous Continuous
Dosing intervals < 5 min < 15 s Continuous
Effluent Usually fully
nitrified
Nitrified at low
loadings
Nitrified at low
loadings

20. Advantages of Trickling filters
• Simple and reliable process that is suitable in areas where large tracts of
land are not available for a WSP treatment system
• Effective in treating high concentrations of organic material depending
on the type of media used;
• Very efficient in removal of ammonia from wastewater;
• Appropriate for small- to medium-sized communities
• Ability to handle and recover from shock loads
• Relatively low power requirements; They require power for pumping
only and do not need large power-hungry aeration blowers. From motor-
driven rotary distributors are powered by fractional horsepower electric
motors.
• They produce less sludge than suspended-growth systems. The sludge
tends to settle well because it is compact and heavy.
• Level of skill and technical expertise needed to manage and operate the
system is moderate
• The cost to operate a trickling filter is very low

21. Disadvantages of Trickling filters
• Additional treatment may be needed for the effluent to meet strict
discharge standards.
• It is a natural method for wastewater treatment that requires a
considerable amount of space.
• Generates sludge that must be treated and disposed of;
• Regular operators attention is needed;
• Relatively high incidence of clogging;
• Relatively low loadings required depending on the media;
• Limited flexibility and control in comparison with activated sludge
processes. They require high maintenance costs of rotary distributor
center mechanisms. Any maintenance service for the rotary
distribution mechanism would require a crane and complete
removal of the rotary distributor mechanism, guy rods, and arms.
• Potential for vector and odour problems

22. OXIDATION PONDS
• Oxidation Ponds are also known as stabilization ponds or
lagoons. They are used for simple secondary treatment of
sewage effluents.
• Within an oxidation pond heterotrophic bacteria degrade
organic matter in the sewage which results in production of
cellular material and minerals.
• The production of these supports the growth of algae in the
oxidation pond.
• Growth of algal populations allows furthur decomposition of
the organic matter by producing oxygen. The production of
this oxygen replenishes the oxygen used by the heterotrophic
bacteria.

23. • Typically oxidation ponds need to be less than 10 feet deep in
order to support the algal growth. In addition, the use of
oxidation ponds is largely restricted to warmer climate regions
because they are strongly influenced by seasonal temperature
changes. Oxidation ponds also tend to fill, due to the settling
of the bacterial and algal cells formed during the
decomposition of the sewage.

24. 24
• Lagoons or oxidation ponds
– Anaerobic
– Facultive
– Aerobic
• Secondary treatment in rural areas
• Polishing ponds
• Serve 7% of population (1000’s)
– >90% of ponds serve populations <10,000

25. 25
Anaerobic Lagoons
• The primary treatment takes place in the anaerobic pond,
which is mainly designed for removing suspended solids, and
some of the soluble element of organic matter (BOD).
• May be covered
• High-strength WW
– meat processing
– dairy waste
• Temperatures must be high--- 75-82F
• BOD loading 20 # per thousand ft3 per day
• Gaseous end products of CO2 and CH4

26. 26
Facultative Lagoons
• During the secondary stage in the facultative pond most of the
remaining BOD is removed through the coordinated activity of
algae and heterotrophic bacteria.
• Anaerobic (bottom layer) and aerobic (upper)
• Bacteria break down organics
– Nitrogen/phosphorous/CO2
• Algae and reaeration (wind) provides O2
• BOD <30 mg/l in warm weather
• SS usually > 30 mg/l because of algae
• Don’t operate well in cold weather
• Can’t handle industrial ww’s

27. 27
Aerobic Lagoons
• The main function of the tertiary treatment in the maturation
pond is the removal of pathogens and nutrients (especially
nitrogen).
• Completely mixed.
• First-stage treatment of municipal WW
• Pre-treatment of industrial WW
• Basins 10-12’ deep
• Pier-mounted floating mechanical aerators
• No algae
• Odor-free if highly aerated

28. Advantages
• Low operational and maintenance cost.
• lagoons provide effective treatment with minimal threat to the
environment.
• Work well in clay soils where conventional subsurface on-site
absorption fields will not work.

29. Disadvantages
• Lagoons must be constructed in clay soil or be lined to prevent leakage.
• May overflow occasionally during extended periods of heavy rainfall.
• If there are extended periods of overcast windless days, a rare occasion
• Offensive odours may occur for a brief time. Lagoons
• Usually recover rapidly if this occurs.
• Can not be installed on a small lot. Takes up a relatively large space for only one
use.
• Lagoons are not aesthetically acceptable to some people. Some people consider
lagoons unsightly and unsafe.
• As with any other open body of water, there is some potential danger.
• Although lagoons are required to be fenced, this does not always prevent access
by people or pets.