CE 152 - Exam 3

General Purposes of Biological Treatment

• Coagulate and remove non-settleable colloidal solids

• stabilize organic matter

Objectives of Biological Treatment in Domestic Wastewater

• To reduce organic content and inorganic nutrients

• To remove toxic organic compounds
  

Objectives of Biological Treatment in Industrial Wastewater

• To remove or reduce concentration of organic and inorganic compounds

Mechanism of biological treatment

• Uses microorganisms, especially bacteria

  • convert organic matter into various gases and cell tissue

  • O₂, NH₃, and PO₄^3- are nutrients needed for conversion of organic matter

  • cell tissues are then removed thru gravity settling

time required for organisms to acclimate to new environment

Lag phase

cells divide at a rate determined by their generation time & ability to process food

Log-growth Phase

no more substrate or nutrients for growth; growth of new cells offset by death of old cells

Stationary Phase

death rate exceeds production of new cells

Log-death Phase

True or False. Most biological treatment processes are comprised of only one biological population

False. They are are comprised of complex, interrelated and mixed biological populations

Factors of growth curve in each biological population

• Food and nutrients

• Temperature

• pH

• Aerobic or anaerobic

Types of biological treatment processes

• Attached-growth

• suspended-growth

• combined

• lagoon

Type of biological treatment process where microorganisms are attached to some inert medium

Attached-growth processes

examples of Attached-growth processes

• trickling filter

• rotating

• biological contactors
  

Type of biological process where microorganisms are maintained in liquid suspension by appropriate mixing methods

Suspended-growth processes

Example of Suspended-growth processes

• activated sludge

• aerobic and anaerobic digestion
  

Type of biological treatment process that is hybrid of attached and suspended-growth processes

Combined processes

Example of combined process

integrated Fixed Film Activated Sludge (IFAS)

Type of biological treatment process where lagoons and constructed wetlands are classified

Nature-based processes

Type of nature based process of biological treatment where treatment take place in ponds or lagoons

Lagoons

Type of nature based process of biological treatment characterized as manmade engineered wetland for treating wastewater effluent

Constructed wetlands

• nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continuously

• treatment occurs as the wastewater flows over attached biofilm

Trickling filter

provides a surface for the biological slime layer or biofilm to attach and grow

Filter media

• collects treated wastewater and solids (sloughs)
  discharged from the filter media, and transports them to a clarifier

• must be designed to support packing, slime growth and wastewater

• allows air ventilation through the filter media

Underdrain System

flow of wastewater including recirculation applied on unit area of the filter in a day

Hydraulic Loading Rate

amount of 5-day (20°C) BOD, excluding the BOD of the recirculant, applied per unit volume of filter media in a day

Organic Loading Rate

Biological treatment unit arrangement

• Single-stage arrangement

• Two-stage arrangement

Basic operation of the trickling filter treatment process

1. wastewater is distributed over the top area through rotating distributor

2. Air circulation in the void space, by either natural draft or blowers, provides oxygen for the microorganisms growing as an attached biofilm.

3. The organic material present in the wastewater is metabolized by the biomass attached to the medium.

4. the inner microbes lose their ability to cling to the surface of the media.

5. The wastewater then washes the slime off the media and a new slime layer will start to grow.

6. The sloughed off film and treated wastewater are collected by an underdrainage which also allows circulation of air through filter.

7. A portion of (supernatant) settling tank the treated effluent from the secondary is recirculated or returned to the TF.

• suspended growth secondary treatment process which involves production of an activated mass of microbes capable of stabilizing a waste aerobically

• these microorganisms are cultivated in aeration tanks, where they are provided with dissolved oxygen and food from the wastewater.
  

Activated Sludge Process

Basic components of Activated Sludge Process

• Reactor

• Liquid-solids separation

• Activated Sludge

• Recycle system

phases of bacterial growth curve

• lag phase

• log-growth phase

• stationary phase

• log-death phase

where microorganisms responsible for treatment are kept in suspension (agitated/mixed) and aerated

reactor

usually a sedimentation tank/ clarifier

Liquid-solids separation

for maintaining high population of microbes (efficient treatment)

Return activated sludge (RAS)

effect of too much waste activated sludge (WAS)

low number of microbes (less efficient treatment) - Food > Microbes

effect of too little WAS (Waste activated sludge)

large concentration of microbes (effluent overflow); food < microbes; imbalance in microbial population; decrease in the dissolved oxygen levels.

Basic Operation of Activated Sludge Process

1. As the influent enters the reactor or aeration tank, the activated sludge microbes consume the solids and organic matter present in the wastewater.

2. MIXED LIQUOR, is agitated and aerated in the aeration tank.

3. Microorganisms are mixed thoroughly with the organics under conditions

   that stimulate their growth

4. The individual microorganisms will then floc together (due to mixing) to form an active mass of microbes or biological floc called activated sludge.

5. The mixed liquor (WW + AS) flows from the aeration tank to the secondary clarifier to separate the solids and microorganism from water by gravity.

6. Most of the activated sludge that settled at the bottom of the clarifier are returned to the aeration tank.

importance of RAS

to maintain high population of microbes in the aeration tank for rapid breakdown of the organic matter.

importance of waste activated sludge

to create a balance between the available food and the mass of microorganism (F/M ratio) allowed to accumulate in the aeration tank

→ for efficient degradation of BOD or organics

concentration of suspended solids present in the mixed liquor of the aeration tank

Mixed Liquor Suspended Solids (MLSS)

the concentration of the organic or volatile suspended solids in the mixed liquor of the aeration tank. It is used as a measure or indication of the microorganisms (biomass) present.

Mixed Liquor Volatile Suspended Solids (MLVSS)

Process of determining MLVSS concentration

1. A well-mixed sample (w/ volume V) is filtered through a weighed standard glass-fiber filter (W_filter)

2. Filter residue is composed of the MLSS + capillary water

3. The filter residue is oven-dried at 103 to 105°C. The mass of the dried filter residue is the mass of the MLSS; MLSS = (W_oven – W_filter) / V

4. The oven-dried filter residue is placed in a furnace and ignited at 550°C for 15 to 30 minutes. The mass lost in the ignition is the mass of the MLVSS while the remaining residue is composed of the inert matter or solids. MLVSS = (W_oven – W_furnace) / V

Effect of High rate of wasting, high F/M Ratio

organisms are saturated with food; thus, poor treatment efficiency

Effect of low rate of wasting, low F/M Ratio

organisms are starved; thus, more complete degradation of food or BOD5 → longer sludge age Ɵ_c required

index of how well a sludge will settle at a given MLSS concentration.

SLUDGE VOLUME INDEX (SVI)

Result of high SVI

poor settleability

Sludge problems

• bulking sludge

• rising sludge

sludge problem characterized by poor settling and poor compactibility

bulking sludge

types of bulking sludge with respect to cause

• Growth of filamentous organisms:

  • Low conc = provide structure for floc

  • High conc = bulking problems

• Water trapped in the bacterial flow

cause of growth of filamentous organisms

• Low dissolved oxygen

• Low food to microorganism (F/M) ratio

• Low pH

• High sulfides

• Nutrient deficiency

• Excessive grease

A sludge that floats to the surface after apparently good settling (sludge settles to the bottom of the clarifier, is compacted, and then starts to rise to the surface)

Rising sludge

Solution for rising sludge

• Increase rate of return sludge flow (Qr)

• Decrease mean cell residence time (avoid denitrification)

types of oxidation ponds

1. Aerobic ponds

2. Aerated lagoons

3. Facultative ponds

4. Anaerobic ponds

5. Maturation or tertiary ponds

type of oxidation pond characterized as:

• Shallow ponds; < 1 m deep

• DO maintained throughout depth (O₂ supplied by algal photosynthesis)

aerobic ponds

type of oxidation pond characterized as:

• Ponds oxygenated by surface or diffused air aeration

Aerated lagoons

type of oxidation pond characterized as:

• 1-2.5 m deep

• Aerobic upper zone (photosynthesis, surface re-aeration), facultative middle zone, anaerobic lower zone

Facultative ponds

type of oxidation pond characterized as:

•Deep ponds that receive high organic loadings (to deplete all O2); anaerobic conditions throughout

Anaerobic ponds

type of oxidation pond characterized as:

• Used for polishing effluents from biological processes (tertiary treatment)

• DO is furnished through photosynthesis & surface re-aeration

Maturation & tertiary ponds (polishing pond)

minimum requirements in the design of oxidation ponds

• a berm or embankment

• impermeable soil

• a fence surrounding the pond

• area around pond mowed and tree growth restricted

• a windbreak

• two to five feet deep

• sensitivity to nearby

• streams and rivers