EGCE 582 FINAL

Optimum pH and operating pH of coagulation

5.5 to 6.5 5-8

LSI<0 LSI=0 LSI>0

the solution is unsaturated with CaCO3 the solution is at equilibrium solution is supersaturated with CaCO3

attached/ fixed growth process

the trickling filter consists of a bed of coarse material, such as stones or plastic material over which WW is applied. BC the MO that biodegrade the waste form a film on the media

anaerobic decomposition

no oxygen or nitrate H2S gas is formed yields CO2, methane and water ammonia, H2S and mercaptans are additional byproducts

waste stabilization pond

now used to refer to all pond or lagoon used to treat organic waste by biological and physical processes

if vs greater or equal to Vc how many particles remain in the tanks

100%

What oxygen transfer efficiency can be achieved?

15-25% with fine-bubble diffusers depending on depth of submergence

if vs < vc how many particles remain in the tank?

0%

Facultative ponds

1-2.5m deep aerobic, facultative, anaerobic

eutrophication

A process by which nutrients, particularly phosphorus and nitrogen, become highly concentrated in a body of water, leading to increased growth of organisms such as algae or cyanobacteria. as algae die and decompose, high levels of organic matter deplete the water of oxygen, causing the death of other organisms

Trickling filter variables E1 Q Cin V F R

E1: fraciton of BOD5 removal for the 1st stage Q: WW flowrate (m3/s) Cin: influent BOD5 (mg/L) V: Volume of filter media (m3) F: recirculation factor R: recirculation ratio

Q Xo V um S X Ks kd Qw Xe Xr

Q: WW flowrate into aeration tank (m3/d) Xo:microorganism concentration entering tank (mg/L) V: volume of aeration tank (m3) um: max growth rate constant (1/d) S: soluble BOD5 in aeration tank and effluent (mg/L) X: microorganism concentration in the tank (mg/L) Ks: half velocity constant (BODs conc. at half the max growth rate)(mg/L) kd:decay rate of microorganisms (1/d) Qw: flow rate of liquid containing microorganisms to be wasted (m3/d) Xe: microorganism conc. (VSS) in effluent from secondary settling tank (mg/L) Xr: microorganism concentration (VSS) in sludge being wasted (mg/L)

Discrete Particle Settling Q, A, Vc

Q: flowrate (m3/s) A: surface of the sedimentation basin (m2) vc= particle settling velocity (m/s)

Mean cell residence time

SRT or theta solids retention time or sludge age average amount of time that microorganisms are kept in the system

Settling Column Test

a column of any diameter, but equal height to the depth of the proposed tank solution should be introduced to ensure uniform distribution of particle sizes occurs from top to bottom the duration of the test should be equivalent to the settling time in the proposed tank at various time intervals, samples withdrawn and percent removal is computed and plotted as a number against the time and depth curves of equal percent removal are drawn settling velocity is calculated

amphoteric

a substance that can act as both an acid and a base

How much lime is required to precipitate the phosphorous in wastewater?

about 1.4-1.5 times the total alkalinity expresses as CaCO3

What happens in secondary clarifier?

activated sludge is settled out some sludge returned and some wasted about 8 m3 of air is provided for each m3 of wastewater treated

Role of chemical unit processes

added substance-increase of TDS destruction process handling, treatment and storage issues increase of sludge cost of chemicals

What facilitates removal of Phosphorous?

addition of Ferric chloride, alum or lime

Sludge treatment process

aimed at separating large amount of water from the solid residue thickening stabilization conditioning dewatering reduction

**Activated Sludge**

biological mass formed when air is continuously injected into WW. Microorganisms are mixed thoroughly with the organic compounds under conditions that stimulate their growth through the use of organic compounds as food. As the microorganisms grown and are mixed by the agitation of the air, the individual organisms clump together (flocculate) to form an active mass of microbes

How is nitrogen removed?

biological- maintaining cell detention time of 15 days chemically- speciating to ammonium ion to ammonia gas -raising pH

Monod's Model

biomass production is limited by the rate of enzyme reaction involving food compound that is in shortest supply relative to its needs

Why are chemicals added to floatation

change the air-liquid and solid-liquid interfaces create surface for absorb or entrap air

destabilization mechanisms

charge neutralization charge reversal double layer compression polymer bridge formation entrapment

Principle chemical unit processes used for wastewater treatment

chemical coagulation chemical precipitation chemical oxidation advanced oxidation photolysis (UV) chemical neutralization scale control and stabilization

Disadvantages of dechlorination

chemical de-chlorination can be difficult to control when near zero levels of residual chlorine are required significant overdosing of sulfite can lead to sulfate formation, suppressed DO content and lower pH of the finished effluent

Technologies available for the removal of heavy metals

chemical precipitation (most common) carbon adsorption ion exchange reverse osmosis

Desirable characteristics of a coagulant

non-toxic and relatively inexpensive insoluble in neutral pH range- do not want high concentration of metals left in treated water

Fundamentals of Chemical Coagulation

colloidal particles (0.01 to 1 micron) have net negative surface charge van der waal force (attract) brownian motion coagulation-destabilize the colloids to bring them together flocculation- process whereby the size of particles increases as a result of particle collision

chemical formation of struvite

combined removal of ammonium and phosphorous magnesium ammonium phosphate hexahydrate (struvite) MgNH4PO4-6H2O

What is aluminum sulfate?

common coagulant hydrated with 14 moles of water normal range of dosage: 5-50 mg/L

Rotating Biological Contactors

consists of a series of closely spaced discs mounted on a horizontal shaft and rotated while ~50% of each disc is submerged in WW discs are lightweight plastic slime is 1-3mm in thickness on disc

What does CO2 production do?

corrosiveness

Microbial growth kinetics

dX/dt=uX dX/dt is the growth rate of the biomass (mg/Lt) u is the growth rate constant (1/t) X is concentration of biomass (mg/L)

anaerobic ponds

deep ponds that receive high organic loadings such that anaerobic conditions prevails throughout the entire depth

Aerobic decompositions is preferred for what?

dilute waste BOD<500 mg/L decomposition is rapid, efficient and has low odor potential

2 Types of Secondary Treatment

dispersed growth: activated sludge, oxidation ditches, ponds, aerated lagoons, stabilization ponds fixed growth: trickling filters, rotating biological contractors (RBCs)

How does struvite formation happen?

during anaerobic digestion, magnesium, ammonium and phosphate are released from the digestion of primary and waste activated sludge if conc of soluble Mg, NH4 and PO4 exceed solubility limit for the formation of struvite at a given pH, crystals of struvite will form anything within the curve forms struvite

Trickling filters

easy to operate rotating distribution arm sprays primary effluent over circular bed of rock or other coarse media air circulates in pores between rocks biofilm develops on rocks and MO degrade wastes as they flow past

high rate clarification

employs physical/chemical treatment and utilizes special flocculation and sedimentation systems to achieve rapid settling Europe the addition of an inert ballasting agent (silica sand or recycled chemically conditioned sludge) and a polymer to a coagulated and partially flocculated suspension

Anoxic Decomposition

facilitated by nitrate as opposed to oxygen avoid in final clarifier formation of N gas will cause large globs of sludge to float to the surface and escape from the treatment plant

How is thickening accomplished?

floatation gravity: solids are allowed to settle to the bottom

F/M ratio

food: microorganisms used in regulating the performance of the activated sludge process controlled by wasting part of the microbial mass high F/M ration yields organisms that are saturated with food, poor efficiency

Aeration System

functioning of aerobic processes such as activated sludge, biological filtration and aerobic digestion depends on the availability of sufficient quantities of oxygen sufficient oxygen to meet requirements of aerobic waste treatment does not enter water through normal surface air-water interfaces

Basic ingredients for biological treatment

high density of microorganisms good contact between organisms and wastes food favorable temp, pH, nutes, carbon source no toxic chemicals present

trickling filter design considerations

hydraulic loading organic loading recirculation ratio

Filtration

if the effluent does not meet the stipulated regulatory criteria filtration process may be used to comply with the stipulated criteria granula membrane carbon

Why recirculate?

increase contact efficiency dilutes the strength increases DO improve distribution over the surface prevent slime from drying out

What does lime due to water?

increases hardness soda ash does not increase harness

Floatation

introduce fine air bubbles to increase buoyant force good for small and light particles (waste activated sludge) chemicals are commonly added

Oxygen Transfer in Clean Water dC/dt=KLa(Cs-C)

kl: overall liquid mass transfer coefficient (L/T) KLa: volumetric mass transfer coefficient (1/T) Ct concentration in liquid build phase at time t (M/L3) Cs: concentration in equilibrium with gas as given by Henry's Law (M/L)

Stages of microbial growth

lag phase, log phase, stationary phase, death phase

What is the benefits of using rectangular vs. circular primary treatment tanks?

limited space, look at site conditions

Pond system design considerations

loading rate should not exceed 22 kg/ha detention time in lagoon should be 6 months

How to remove phosphate?

may be incorporated into either biological solids or removed by addition of chemicals principle chemicals: aluminum, ferric iron, calcium

Problems with phosphorous in WW

may cause algae growth eutrophication

What happens as the discs rotate in RBC?

microbes adhere to rotating surface and grow rotating disc aerates WW, adding O2, mixing treated and partially treated WW as attached microbes pass through the reservoir, they absorb other organics for breakdown

Mixed liquor

mixture of activated sludge and wastewater

How to avoid problems associated with rock filters

modules of corrugated plastic sheets and plastic rings are popular -larger surface area -increased void ratio -lighter, so can be taller rock filters can only be 3m high but synthetic media filters can be 12m high

What form is phosphate in WW and how is it typically removed?

mono-hydrogen phosphate (HPO4 2-) precipitation

Coagulants/ Flocculents

natural and synthetic organic polymers metals salts (alum, ferric sulfate) pre-hydrolyzed metal salts (PACl, PICl) hydrous polymer

What is flocculation dependent on?

opportunity for contact, which varies with overflow rate, depth of the basin, velocity gradients in the system, concentration of particles and range of particle sizes

oxidation pond vs sewage lagoon

ox pond: received partially treated WW lagoon: raw wastewater

Flocculent Settling

particles in relatively dilute solutions will not act as discrete particles, but will coalesce during sedimentation as coalescent or flocculation occurs, the mass of the particle increases and it settles faster

Primary Treatment

physical: driven by gravity remove ~55% SS ~35%BOD t~ 2 hrs SOR = 1000 gpd/ft2 weir loading = 20,000 gpd/ft2 D ~10ft

aerated lagoon

ponds oxygenated through the action of surface or diffused air aeration

Waste Activated Sludge WAS

portion of the microorganisms discarded too much wasting will result in low concentration of microorganism to effectively treat wastewater too little wasting will result in very high concentration leading to eventually spilling over

How does precipitation of metals in wastewater work?

precip as metal hydroxides through addition of lime precip as metal sulfides

Advantages of de-clorination

protects aquatic life prevents formation of harmful chlorinated compounds in drinking water

Determining the overall oxygen transfer coefficient in clean water

removal of DO from known volume of water by the addition of sodium sulfite followed by re-oxygenation to near the saturation level DO of water volume is monitored during reaeration period by measuring DO concentrations at several different points

Major purpose of secondary treatment

remove soluble BOD that escapes primary treatment further remove SS

RAS

return activated sludge (20-30% of WW flow)

recirculation ratio

return of a portion of the effluent to flow through the filter ratio of returned to incoming flow

Tertiary treatment

secondary treatment does not adequately remove the pollutants can be tailored for specific circumstances nitrogen, phosphorous and heavy metals are a major concern

Discrete Particle Settling Objective

select particle with a terminal velocity Vc and to design basin so that particles with terminal velocity equal to or greater than Vc will be removed

aerobic ponds

shallow (<1m) DO maintained naturally

EPA recommendations for phosphates

should not exceed 0.05 mg/L in a stream at a point where it enters lake or reservoir should not exceed 0.1 mg/L in streams that do not discharge directly into lakes or reservoirs

microbes of interest

single-celled bacteria rotifers (eat bacteria, fungi, plankton, protozoa) protozoa: eat bacteria, enhance floc formation, removal of fecal coliforms

What must be considered in terms of discharging BOD5 levels?

some fraction of the suspended solids that do not settle in the secondary tank also contribute to the BOD5 load to the receiving body to achieve desired effluent quality both the soluble and insoluble fractions of BOD5 must be considered

What happens in secondary treatment?

stabilization of organic matter by microorganisms convert colloidal and dissolved carbonaceous organic matter into various gases and protoplasm (C6H1ON2) protoplasm has higher SG than water and can be removed from the treat liquid by gravity settling

What is most commonly used for dechlorination?

sulfur dioxide

Scour Velocity

to avoid the re-suspension (scouring) of settled particles, horizontal velocities through the tank should be kept low

True or false: not a single pure culture of microbes exists in the environment

true

True or false: polymers are more likely to be used as coagulant aids, not primary coagulant

true

True or False: the velocity of water decreases as the water flows upward

true vs: overflow rate/ hydraulic surface loading

Monod Equation

u= (umS)/(Ks+S) Ks: half saturation constant (mg/L) um: max growth rate constant (1/t) S: conc. of limiting biomass (mg/L) multiply by 1/Y to account for food not converted to biomass

Trickling filters limitations

under high organic loading, slime may clog the void causing flooding and failure of the system void spaces in the filters restrict the circulation of air and the amount of oxygen available eventually these issues could restrict the amount of WW that could be processed

Advantages of high rate clarification

units are compact rapid start up times to achieve peak efficiency highly clarified effluent is produced

Biological treatment

use microorganisms to convert organic wastes into stabilized similar to self-purification process in streams secondary treatment

Langelier Saturation Index

used to predict whether CaCO3 will DoD LSI = pH - pHs pH is the actual hydrogen ion concentration pHs is the pH at saturation pHs = pCa2+ + pAlk + C

Disinfection

usually with chlorine for 15 mins contact time in detention basin WW is then dechlorinated to a level defined in NPDES permit

Particle settling velocity Terminal settling velocity

vc vp or vs

What value to use for up flow clarifier design?

vc=0.70vs

How to calculate settling velocity from column test?

vc=H/tc H= height of the column tc: time required for given degree of removal to be achieved

Scour Velocity vh k s g d f

vh: horizontal velocity that will just produce scour (m/s) k:constant that depends on type of material being scoured s:specific gravity of particles g:acceleration due to gravity (9.81 m/s2) d: diameter of particles f: darcy-Weisbach friction factor

Three steps to anaerobic decomposition

waste components are hydrolyzed complex organic compounds are fermented organic acids are converted to methane

How is ASP controlled?

wasting a portion of the microorganisms each day in order to maintain the adequate amount of microorganisms

Overflow Rate

water flowing over the top of the sedimentation tank into the weir system (surface loading rate or liquid velocity)