Wastewater: Primary Treatment

Settlement is the oldest and most common method in

primary treatment systems, removing solids larger than

10 mm (0.39 inches).[2] This method removes 35% of

biological oxygen demand (BOD) and 60% of total

suspended solids (TSS) in meat and vegetable processing

wastes.[2] Solids smaller than 1 mm (0.04 in.) do not settle

by gravity and must use chemical treatments to settle.[2]

Soluble and colloidal foods like soft drinks and salad dres-

sing waste do not settle out well either.[2] Stokes’ law is the

basis for settlement theory, calling on the density differ-

ences between the particles and water, flow velocity, and

temperatures.[1,2] The density differences are one of four

types of settlement behavior noticed.[1,2] The second is

where settlement particles have greater velocity than the-

ory states where the flocculent settles faster than the tradi-

tional settlement behavior is formed.[1,2] The remaining

two settlement behaviors, zone settlement and compressive

settlement, occur when activated sludge is concentrated

in the system.[1,2]

Types

The two types of settlement tanks used are circular and

rectangular. Most tanks are circular because they are

cheaper and havemore reliable scrapers, though rectangular

tanks are common to reduce the required operational land

area.[2] These tanks are designed for low velocity and uni-

form flow, with sidewall depth of 2.4-4.5 m (7-15 ft) with

slope to bottom of 5-10.[1,2] The tank is traditionally made of reinforced concrete though prefabricated tanks are made

either of plastic or of steel depending upon tank volume.[2]

The scrapers usually are made of galvanized steel, and

the weirs used for uniform flow within the tank are made

of plastic.[1,2] A hydrostatic head periodically removes

any accumulated sludge with decanting through a screw-

threaded bell valve.[2] Extra removal of fatty wastes

uses a screen baffle and upper scraper to combine the

Loading Rates

The common loading rates for settlement tanks are shown

in Table 1.[2]

FLOTATION

Introduction

A large variety of food processing wastewaters contain

fat, oil, and grease (FOG).[1,2] Most FOG is biodegradable

if soluble or dispersed to even concentrations for bacteria

usage.[2] The rest separates from the main wastewater,

creating problems for bioreactors, process instruments,

and aeration gas transfer.[2] For aerobic treatments, the

maximum concentration is 150 mg L-1.[2] To reach this

standard, flow is reduced in a single chamber and oil

is trapped in underwater weirs, causing it to float to the

surface.[2] If the wastewater has detergents or surface-

active chemicals in them FOG emulsifies and separation

efficiency is reduced.[2] Flotation overcomes this in three

ways: fat traps; dissolved air flotation (DAF); and inclined

plate separators.[2]

Fat Traps

Fat traps are designed for even flow through a tank,

allowing density differences to raise fat to the surface

without disturbing any sludge and scum already settled

at the bottom.[2] A drain valve removes the built-up

sludge and scum at the tank bottom. Fat traps are designed

similar to settling principles.[2] Their typical length/width

ratio is 2:1. Table 2 shows the fat trap load rates.[2]

Dissolved Air Flotation

Dissolved air flotation, like sedimentation, is particle size

dependent and also depends upon gas transfer efficiency

and bubble attachment to the particle.[1,2] This lowers the

required tank size and avoids any potential anaerobic con-

ditions.[2] Compared to gravity separation, DAF is more

complex and less reliable.[2] DAF uses pressure release

from a supersaturated solution of air.[1,2] The most common

DAF is where pressure supersaturates the wastewater and is

released to remoisten the dissolved air.[2] Design is based

on flow velocity and retention, but also on a minimum

air/solids ratio. (The last is difficult to set by theory and is

optimized prior to commissioning.)[2] Many DAF units

are purpose built for great flexibility, requiring periodic

adjustment.[1,2] Typical DAF operating characteristics

are 30-60 m3 m-2 day-1(98.4-196.8 ft3 ft-2 day-1) sur-

face load, 20-30 minutes retention time, 25-100% recycle

rate, and 3-6 atm (43.1-86.2 psig).[2] Problems with DAF

include poor reliability and higher operational costs than

gravity separation, according to operational surveys.[2]

DAF is also not suitable to the constant changes in waste-

water characteristics.[2] Table 3 shows the ideal DAF

performance compared to gravity separation.[2]

Incline Plate Separators

Incline plate separators are settlement tanks with a series

of incline parallel plates inside a tank.[2] The flotation is

new to treating food-processing wastewaters, so limited

research has been done on this.[2] Startup costs for inclined

plate separators are less than conventional settling tanks

though maintenance and sanitation costs are higher.[2]

Inclined plate separators use the hydrophobic interface

of surfaces and particles to allow them to coalesce on the

separator’s surface.[2] The result is a smaller tank size, with

retention time of 30 min and increased surface loads of

2.5-3.0 m3 m-2 hr-1(8.10-9.84 ft3 ft-2 hr-1).[2]

CHEMICAL TREATMENT

Introduction

Chemical treatment in primary treatment systems are used

to correct pH and improve settling rates with increased

particle density.[2] It is used when an upset condition

or seasonal changes occur.[2] Chemical treatment’s big

disadvantages are chemical costs, good control to optimize

performance, and higher operational and maintenance

costs.[2] The four most common methods are: pH neutraliz-

ing; coagulation/flocculation; oxidation; and nutrients.[2]

pH Neutralization

pH neutralization’s cheapest and most common chemicals

are sulfuric acid and sodium hydroxide, each sold at a

maximum 50% v/v solution.[2] Other products considered

may be lime (cheaper base, but poor solubility and more

difficult to prepare), commercial slurries, and phosphoric

acid and carbonic acid for anaerobic treatments in lieu of

sulfuric acid.[2]

Coagulation/Flocculation

Coagulation is used for very small, positively charged

particles less than 5 mm (0.2 inches) to absorb negatively

charged ions in the mixing solution.[1,2] Flocculation

bridges these small particles together into larger particles

to create a sludge blanket by straining and enmesh-

ment.[1,2] Polymers used for flocculation include polyacri-

mide, aluminum hydroxide, and ferric hydroxide, though

they may not be needed because of the great mixing

between the chemicals and the particles.[2] Fig. 1 shows a

typical flocculation system.[2] A typical flocculation sys-

tem requires a rapid mix tank to completely mix the waste

and the reagents.[1,2] This forms the flocculant that is

transferred to a flocculation tank for gentler mixing to

ensure good enmeshment.[2] This gentle mixing is done

in a second, slow-stirring flocculation tank (mixer speed

of 1-2 rpm) before the flocculant’s final settlement or

flotation.[2] To ensure correct flocculant dosage and

conditions, lab and pilot plant testing is required.[1,2]

Cost-benefit analysis of chemicals may be required for

sludge consolidation.[2] Sufficient polymer concentrations

are 1-5 mg L-1 organic and/or 10-100 mg L-1 of iron or

aluminum-based compounds.[2] If there are recycled solids

involved, food-grade flocculants like cellulose, starch,

lignosulfonic acid, and calcium polyphosphate would

be used.[2]

Oxidation

Oxidation is used during a very short operating season

or when a short-term problem occurs, like toxic wastes

or a biological malfunction.[1,2] It is also useful when

there is not enough aeration.[2] Pure oxygen, ozone,

chlorine, and permanganate are usually used during

oxidation.[2] If wastewater is being recycled, then chlor-

ine, hypochlorite, and ozone are used as wastewater

Nutrients

To improve the performance of some wastes, supple-

ments of nitrogen, phosphorous, trace metals, and vita-

mins are added.[2] For successful aerobic treatment, a

carbon (BOD), nitrogen (ammonia-based), and phos-

phorous ratio of 100:5:1 is established.[1,2] If organic

nitrogen is in this as a protein, the microbes in the waste-

water can use it as a nitrogen source.[2] Urea is added only

if nitrogen is required. If both nitrogen and phosphorous are

needed, inorganic fertilizer is added.[2] When biomass

requirements are met, the nutritional need is lowered

because the recycled nutrients are controlled by ensuring a

1-2 mg L-1 residue of nutrients in the treated wastewater.[2]

For anaerobic bacteria, the carbon-nitrogen-phosphorous

ratio is 300:5:1 because of the lower growth rate and nutri-

tional requirements.[2] Anaerobic bacteria are vulnerable to

shortages of iron, cobalt, nickel, and manganese though

all are commercially available and commonly added to

industrial digesters.[2] Extra nutrients also assist aerobic

treatment with complex nutrients and in addition are an

effective method in countering any mild sulfur toxicity in

the system.[2]

CONCLUSION

Primary treatment systems use settling, flotation, and che-

mical treatments to reduce BOD, TSS, and FOG in food

processing wastes in order to keep a facility in compliance

with local, state, and federal regulations. As environmental

regulations tighten worldwide, the primary treatment

systems must adapt to meet the challenges ahead.