Quiz 2: Wastewater Treatment Study Guide

Recognize approximate influent characteristics of municipal wastewater in terms of BOD_5, TP, and TDS. (in terms of mg/L)

Biological Oxygen Demand, BOD_5: 100-350 mg/L

Total Phosphorus, TP: 5-20 mg/L

Total Dissolved Solids, TDS: 250-1,000 mg/L

What is the primary objective of preliminary treatment (see Vesilend)?

Physically remove large solids to prevent damaging later unit operation and to disinfect waste.

-Removes....

....60% solids

....30% oxygen demand

....20% phosphorus

What methods are used in preliminary treatment?

Screening: filter

-Removes large floating objects (e.g., rages, shoes, etc.) that might damage/clog pipes and is made steel bars with mesh.

Comminutor: circular grinder

-Grinds coarse material into smaller pieces that can be left wastewater flow, avoiding problems of materials collected in screen.

Grit Chamber: settle

-Holds wastewater for its detention time = volume/flow rate ~20-30 s. to allow heavy materials like grit and sand to settle but NOT organic matter, disposing waste to landfills.

What is the removal mechanism used in a primary clarifier/primary settling tank/sedimentation?

-Sedimentation: flow speed is reduced to allow suspended solids to settle by gravity

-Primary Sludge/Raw Sludge: settled suspended solids

-Disinfectant: Chlorine & UV light

-Size = overflow rate & detention time; circular & rectangular

-Detention time: 1.5-3 hours

What removal efficiencies are typical of primary clarifier/primary settling tank/sedimentation?

Removes...

...50-65% suspended solids

....25-40% BOD

What is the main purpose of secondary treatment?

Remove BOD from dissolved organics through microbial action.

What is the main removal mechanism in secondary treatment?

Microorganism convert organic waste into stable compounds.

1) Suspended Growth Treatment: microorganisms suspended & move with water

-Activated Sludge: injected oxygen for increased microbial mass

-Return Activated Sludge: secondary clarifier settles sludges from water through sedimentation & is returned

-Waste Activated Sludge: to maintain steady state, a portion must be wasted

-Membrane Bioreactors: Flashcard #14

-Aerated Lagoons & Oxidation Ponds: Flashcard #16

2) Attached Growth Treatment/Fixed Film Reactors: water flows past fixed microorganisms

-Trickling Filter: sprats the wastewater over a circular bed of plastic/coarse material & biological slime absorbs waste

-Rotating Biological Contractor (RBC): alternative to trickling filter, acting as a sprinkler

What are the minimum required and typical removal efficiencies for BOD_5 in secondary municipal wastewater treatment?

Required BOD: 85%,

Required BOD_5: 30 mg/L

Typical BOD: 90%

What are the typical hydraulic retention times in a grit chamber, primary clarifier, and anaerobic digester? [Note that these are significantly different.]

Grit Chamber: 20-30 seconds

Primary Clarifier 1.5-3 hours

Anaerobic Digester: 10-15 days

What are the different purposes of a primary clarifier versus secondary clarifier?

Primary Clarifier: removes solids through sedimentation before biological treatment

Secondary Clarifier removes BOD through activated sludge after biological treatment

What is the 'mixed liquor' in activated sludge system?

It is the combination of both sludge and water created in primary clarifier.

Why is sludge returned to aeration basin?

To maintain sufficient bacteria for continuous treatment

With respect to where bacteria 'live,' how do activated sludge systems differ from trickling filters?

Activated Sludge: bacteria lives in the reactor effluent, which is the mixture of wastewater and microbial cells (mixed liquor)

-Suspended Growth

-Requires high energy

-Higher treatment efficiency

-More complex.

Trickling Filters: bacteria lives on the surface of the packing media (fist size rocks, plastic media)

-Attached Growth

-Requires less energy

-Requires larger area

What are MBR and RBC systems?

Membrane Bioreactor: suspended growth treatment

-Processes mixed liquor through membrane filtration in activated sludge aeration tank, avoiding need for secondary clarifier.

-Very efficient at removing bacteria and cysts (not viruses), making them especially useful in systems where wastewater reuse and reclamation is desired

-More expensive

Rotating Biological Contractor: Attached growth treatment

-Variation of trickling filter where rotating disks are utilized for microbial growth.

-When in water, microorganisms absorb organics.

-When out of water, microorganisms are supplied with oxygen.

What are the purposes of sludge stabilization? (see Vesilend)

To reduce sludge order putrescene that is aesthetically displeasing and potentially harmful.

-Lime stabilization: add lime (hydrated lime Ca(OH)2 or quicklime (CaO) to sludge, raising pH to about 11 or above and reducing odor and destroys pathogens → however, it is a temporary solution as pH drops within days

Where in the system do we typically want aerobic versus anaerobic degradation and why?

Aerobic:

-Decomposition near the surface

-Oxygen comes from surface aeration and algal photosynthesis

Anaerobic:

-Decomposition near the bottom

-No oxygen

Aerated Lagoons and Oxidation Ponds: Similar to eutrophic lakes

Facultative Ponds: a mix of aerobic and anaerobic

What are the outputs from an anaerobic digester?

0.1 mg VSS/mg → BOD5 Biogas (Methane & CO₂): Can be used for electricity or heating.

-Stabilized Sludge: Can be used as fertilizer or disposed of in landfills.

What are some methods for dewatering sludges?

Sand Bed:

-Consist of tile drains in gravel covered in sand and sludge is poured on the bed

-Seepage and evaporation separate the water from the solids

Belt Filters:

-It is a pressure filter and a gravity filter.

-Sludge goes through moving belt and free water drips through, retaining the solid and squeezing the sludge between two belts.

Centrifuges:

-Consists of a solid bowl decanter, which is of a bullet-shaped body rotating on its long axis. Sludge is placed into the bowl and then solid settle under a centrifugal force.