Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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0024 Breakfast is usually followed by midday and

evening meals. Traditionally, many cultures con-

sumed three hot meals per day; now many cultures

eat only one hot meal per day. Cultures also vary in

which meal of the day is considered the main meal,

which is usually heavier and is usually served hot. In

some cultures, the main meal is the midday meal, and

in others, it is the evening meal. While some cultures

have a more narrow definition of what constitutes a

main meal (for example, the ‘proper meal’ of the

English consisting of ‘meat and two veg’), other cul-

tures have a broader definition of main meals. The

development and spread of pizza meals, sandwich

meals, and salad meals is gradually changing how

we view meals.

0025 De Castro studied meal patterns extensively using

a diet-diary technique, and extended this work to a

cross-cultural comparison of France, The Nether-

lands, and the USA. While the study is limited to

small numbers of university students, the results

portray interesting differences and similarities across

cultures. Compared with the French and Americans,

who are more similar, the Dutch students consumed

more food, had more meals with shorter intervals,

ate with more people present, and ate for longer

durations at a slower rate. The Americans tended to

eat bigger meals as the day proceeded, a behavior that

was even more pronounced in the Dutch, whereas the

French ate their biggest meal in the early afternoon.

However, the researchers concluded that the similar-

ities among cultures are stronger than the differences,

suggesting underlying mechanisms for food intake

and meals. They also cautioned that differences in

methods can lead to differences in results. The spread

of the global economy is increasing the need to better

describe and understand cultural differences in eating

in order to identify opportunities for broader distri-

bution of food products and food service.

0026 There is also interest in continual snacking and

meal-taking, often called grazing. A Symposium on

Assessing Eating Patterns presented a cross-section of

relevant research on meals across many cultures. One

study reported snacks as a percentage of total eating

in eight European countries: the numbers ranged

from 6 to 32% for women and from 7 to 31% for

men. Studies in both Greece and Germany showed

midmorning, midafternoon, and evening snacking in

addition to the regular three-meal pattern. In Ger-

many, snacks accounted for 20% of energy intake,

and Swedish data indicated even higher percentages

of daily intake in the form of snacks. In a national

sample of young adults conducted in the USA,

snacking prevalence was shown to have increased

significantly from 77 to 84% over the past 25 years.

The nutritional contribution of snacks to total daily

energy intake went from 20 to 23%, likely due to

a 26% increase in energy consumed per snacking

occasion and a 14% increase in the number of

snacks taken per day. Intake of high-fat dessert snacks

has decreased, but the intake of high-fat salty

snacks has doubled during the same time period.

0027A final set of factors that have been studied for

their effects on food choices and intake is the in-

creased emphasis on health, diet, and slimness in the

past decades, all of which have prompted meal

patterns centered on restrained eating. Although

restrained eating normally can produce short-term,

often cyclic changes in meal patterns among other-

wise healthy individuals, there is a danger that the

desire for being thin can lead to eating disorders. But

the dangers of these restrained eating patterns are not

limited to individuals with eating disorders.

0028The field of human eating habits is very complex

and involves many disciplines, only some of which

have been covered in this section. The study of human

eating habits is expanding rapidly within each discip-

line and also between disciplines, as the connections

between different aspects of human eating habits are

being investigated. In addition, the relationship of

eating habits to health and fitness is receiving more

and more attention, and eventually we will come

to understand more fully how eating habits affect

general well-being.

See also: Dietary Surveys: Measurement of Food Intake;

Surveys of Food Intakes in Groups and Individuals; Food

Acceptability: Affective Methods; Market Research

Methods

Further Reading

DeCastro JM, Bellisle F, Feunekes GIJ, Dalix A-M and

deGraaf C (1997) Culture and meal patterns: A com-

parison of the food intake of free-living American,

Dutch and French students. Nutrition Research 17:

807–829.

Drewnowski A (1997) Taste preferences and food intake.

Annual Review of Nutrition 17: 237–253.

Furst T, Connors M, Bisogni CA, Sobal J and Flak LW

(1996) Food choice: a conceptual model of the process.

Appetite 26: 247–266.

Hedderley D and Meiselman HL (1996) Modeling meal

acceptability in a free choice environment. Food Quality

and Preference 6: 15–26.

Herman CP and Polivy J (1975) Anxiety, restraint, and

eating behavior. Journal of Abnormal Psychology 84:

66–72.

Marshall D (ed.) (1995) Food Choice and the Consumer.

Glasgow: Blackie Academic and Professional.

Meiselman HL (1988) Consumer studies of food habits.

In: Piggott JR (ed.) Sensory Analysis of Foods, 2nd edn,

pp. 267–334. London: Elsevier.

EATING HABITS 1967

Meiselman HL (ed.) (2000) Dimensions of The Meal: The

Science, Culture, Business and Art of Eating. Gaithers-

berg, MD: Aspen.

Meiselman HL and MacFie HJH (eds) (1996) Food Choice

Acceptance and Consumption. Glasgow: Blackie Aca-

demic and Professional.

Oltersdorf U, Schlettwein-Gsell D and Winkler G (1999)

Assessing eating patterns – an emerging research topic in

nutritional sciences: introduction to the symposium.

Appetite 32: 1–7 (followed by symposium papers pp.

8–112).

Pliner P and Hobden K (1992) Development of a scale to

measure the trait of food neophobia in humans. Appetite

19: 105–120.

Shepherd R (1989) Factors influencing food preferences

and choice. In: Shepherd R (ed.) Handbook of the Psy-

chophysiology of Human Eating, pp. 3–24. Chichester,

UK: Wiley.

Stunkard AJ and Messick S (1985) The three-factor eating

questionnaire to measure dietary restraint, disinhibition

and hunger. Journal of Psychosomatic Research 29:

71–83.

Van Strien T, Frijters JER, Bugers GPA and Defares PB

(1986) The Dutch eating behavior questionnaire for the

assessment of restrained eating, emotional and external

eating behavior. International Journal of Eating Dis-

orders 5: 293–315.

Van Trijp HCM and Steenkamp J-BEM (1992) Consumers’

variety seeking tendency with respect to foods: measure-

ment and managerial implications. European Review of

Agricultural Economics 19: 181–195.

Zizza C, Siega-Riz AM and Popkin BM (2001) Significant

increase in young adults’ snacking between 1977–1978

and 1994–1996 represents a cause for concern! Prevent-

ive Medicine 32: 303–310.

Eels See Fish: Introduction; Catching and Handling; Fish as Food; Demersal Species of Temperate Climates;

Pelagic Species of Temperate Climates; Tuna and Tuna-like Fish of Tropical Climates; Demersal Species of

Tropical Climates; Pelagic Species of Tropical Climates; Important Elasmobranch Species; Processing;

Miscellaneous Fish Products; Spoilage of Seafood; Dietary Importance of Fish and Shellfish; Fish Farming; Fish

Meal

EFFLUENTS FROM FOOD PROCESSING

Contents

On-Site Processing of Waste

Microbiology of Treatment Processes

Disposal of Waste Water

Composition and Analysis

On-Site Processing of Waste

E L Stover and C K Campana, Stover & Associates,

Inc., Stillwater, OK, USA

Introduction

0001 Biological waste water treatment is the primary

method of preparing food-processing waste water

flows for return to the environment. Increasing

waste water loads on existing plants and more

stringent government discharge requirements have

put considerable pressure on the food-processing in-

dustry to refine and understand better the design and

management of biological waste water treatment pro-

cesses. Though activated sludge and other biological

treatment processes are still frequently operated by

general guidelines and ‘rules of thumb,’ facility design

and operation must be guided by consideration of

both the physical and biological aspects of waste

water treatment. Various modifications and combin-

ations of aerobic and anaerobic biological treatment

processes are commonly used in the food-processing

industry.

1968 EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste

Biological Treatment Processes

0002 Biological treatment processes, whether aerobic or

anaerobic, are typically divided into two categories:

suspended-growth systems and fixed-film systems.

Combined or coupled fixed-film and suspended-

growth systems are also commonly used for food-

processing waste water treatment. These processes

can be batch, semicontinuous, or continuous-flow

processes. Suspended-growth systems are more com-

monly referred to as activated sludge processes, of

which several variations and modifications exist.

The basic system consists of a large basin into

which the contaminated water is introduced, and

air or oxygen is introduced by either diffused aer-

ation or mechanical aeration devices. The microor-

ganisms are present in the aeration basin as

suspended material. After the microorganisms

remove the organic material from the contaminated

water they must be separated from the liquid

stream. This is normally accomplished by gravity

setting. After separating the biomass from the

liquid, the biomass increase resulting from synthesis

is wasted and the remainder is returned to the aer-

ation tank. Thus, a relatively constant mass of

microorganisms is maintained in the system. The

performance of the process depends on the recycle

of sufficient biomass. If biomass separation and

concentration fail, the entire process fails. (See Ef-

fluents from Food Processing: Microbiology of

Treatment Processes.)

0003 Another type of suspended-growth system is

the aerobic thermophilic treatment process. High-

temperature and/or high-strength waste streams as

well as waste biological sludges produced during

mesophilic biological treatment are excellent candi-

dates for aerobic thermophilic digestion. This process

uses natural digestion to stabilize wastes in the 50–



C temperature range. A combination of mixing

intensity control and oxygen injection rate are util-

ized to control the reaction rate, oxygen transfer rate,

heat loss, and resultant bulk liquid temperature. The

air (oxygen) is injected at the bottom of the enclosed

reactor vessel, flows up through the bulk liquid, and is

collected in the head space of the reactor. Major

advantages include significant improvement over

mesophilic treatment relative to chemical oxygen

demand (COD) removal and volatile solids destruc-

tion at significantly lower hydraulic and solids reten-

tion times. Total solids reductions of 50–60% have

been observed with waste biological sludges. The gen-

eral requirements to maintain appropriate thermo-

philic temperatures include sufficient biodegradable

organics (COD and/or volatile suspended solids) to

provide heat of oxidation, an insulated reactor, and

adequate mixing and oxygen transfer efficiency to

minimize excessive heat loss.

0004Fixed-film biological processes differ from sus-

pended-growth systems in that microorganisms attach

themselves to a medium that provides an inert sup-

port. Biological towers (trickling filters), rotating

biological contactors, and anaerobic reactors are the

most common forms of fixed-film processes. Bio-

logical towers are a modification of the trickling filter

process. The medium, which is normally comprised of

polyvinylchloride (PVC), polyethylene, polystyrene,

or redwood, is stacked into towers which typically

reach 4.6–6.1 m (16–20 ft) high. The contaminated

water is sprayed across the top and, as it moves

downward, air is pulled upward through the tower.

Forced air ventilation is also commonly used to insure

adequate air flow and oxygen transfer from the gas

phase to the bulk liquid. A slime layer of microorgan-

isms forms on the medium and removes the organic

contaminants as the water flows over the slime layer.

0005A rotating biological contactor (RBC) consists of a

cylinder arrangement of plastic media, connected by

a shaft, set in a basin or trough. The contaminated

water passes through the basin where the micro-

organisms, attached to the media, metabolize the

organics present in the water. Approximately 40%

of the media surface area is submerged. This allows

the slime layer to come into contact with alternately

the contaminated water and the air where oxygen is

provided to the microorganisms.

0006The combined or coupled fixed-film and suspended-

growth processes offer many advantages for treat-

ment of medium to high-strength food-processing

waste waters when compared to use of either technol-

ogy alone. The coupled process typically requires

smaller land areas, and typically will have lower

initial construction costs than a process consisting

of a suspended-growth system alone. Electrical

power costs for coupled systems are typically lower

compared to comparable systems using only the

suspended-growth process. Process control is usually

less demanding because the roughing treatment

provided by the fixed-film system helps dampen

the organic loading variations on the suspended-

growth reactor. When coupled systems are employed

for nitrification, much of the organic loading will

be removed in the fixed-film system, allowing the

suspended-growth system to achieve stable, reliable

nitrification.

0007There are several types of anaerobic reactors that

incorporate into the design a relatively high sludge

retention time (SRT) to account for the slow cell

growth rate associated with anaerobic organisms.

Anaerobic filters (AF), upflow anaerobic sludge blan-

kets (UASB), and the Hybrid upflow sludge bed filter

EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste 1969

process (Hybrid) are three anaerobic configurations

commonly used to achieve and maintain high SRTs.

0008 The AF reactor designs employ certain types of

inert support material on which microbial biofilms

develop as substrate passes through the reactor. This

design enables these processes to operate at very long

biomass retention times. Support or carrier material

varies in size, shape, density, and placement according

to the type of reactor design. The generic term ‘anaer-

obic filter’ usually refers to a fixed-film reactor. These

processes use large support material such as rocks or

plastic media designed for the purpose of forming a

static bed in the reactor vessel. Waste flow through

such media beds may be upflow, downflow, or hori-

zontal. Other fixed-growth anaerobic systems employ

much smaller media with high recycle rates of treated

waste water in order to maintain the bed in a constant

state of motion within the liquid medium in the

reactor. Fluidized-bed reactors use relatively heavy

support media such as sand or activated carbon and

high recycle rates to keep the bed in a completely

fluidized condition. Expanded bed reactors employ

lighter material such as diatomaceous earth or ground

corncobs and lower recycle rates to maintain the bed

in a slightly suspended state. Both systems offer ad-

vantages in avoiding clogging, a major problem of the

fixed-film anaerobic filters. Expanded beds can also

handle suspended solids in the waste feed more read-

ily. Fluidized and expanded beds, with their smaller

media, provide much greater specific surface area for

biofilm attachment. This means that much higher

volumetric loading and conversion rates are possible

than with static anaerobic filters, at the cost of add-

itional pumping for recycle and somewhat complex

operational requirements.

0009 UASB reactors are the most common high-rate

anaerobic treatment process. The process works on

the principle of promoting sludge granulation by

proper seeding with a granular inoculum of anaerobic

bacteria, and adjusting organic loading and upflow

feed rates so that biomass is retained as a dense blan-

ket with a clear liquid zone above. Thus, the system

acts similar to a biologically active sludge clarifier-

thickener. The UASB is operated as a single-pass

system, although waste sludge can be settled and

returned. The exact biological or physical mechan-

isms controlling sludge granulation are still poorly

understood, even though the UASB process is in

successful commercial use. Start-up, however, can

be facilitated by inoculating the reactor with well-

granulated sludge from an operating system.

0010 The Hybrid process has the advantages of both the

upflow AF and the UASB processes while minimizing

the disadvantages. The UASB has upward flow of

waste water through a dense bed of active granular

sludge with good settling properties, and then flows

through a less dense blanket of suspended flocs. The

waste water leaves the reactor via a solids/liquid/gas

separation device at the top of the reactor. This

system can retain high biomass concentrations (and

high SRTs), but is limited by the settling properties of

the granular sludge.

Biological Treatment Process Design

0011Prior to the last decade, and even today, design engin-

eers have employed ‘rule of thumb’ techniques for the

design of secondary biological waste water treatment

processes for food-processing waste waters. Design of

many biological systems has been based on empirical

criteria derived from past experiences and working

plants. Typical design criteria include detention time

and biochemical oxygen demand (BOD) loading per

unit volume. The need for employment of design

models which attempt to describe the functional

behavior of biological treatment processes has been

recognized. Six of these models for the activated

sludge process, which are identified according to the

names of the investigators who had major roles in

their development, are:

0012Eckenfelder

0013McKinney

0014Lawrence and McCarty

0015Gaudy

0016Weston

0017Stover and Kincannon

0018The aim of all of these design models is to provide

more accurate predictive equations which are in keep-

ing with the underlying metabolic and biological

principles governing the waste water purification pro-

cess. These modern concepts for design utilize various

mathematical models which describe relationships

governing microbial growth and substrate utilization

or waste water purification. The two essential elem-

ents of these models are quantitative description of

the properties of the growing biomass (the biological

kinetic constants) and manipulative operating (engin-

eering) variables. Methods have been developed to

obtain the essential biokinetic constants from bench

and pilot-scale investigations.

0019The mechanisms of waste water purification are

the same for the suspended-growth activated sludge

processes and fixed-film media biological filters

and rotating biological contactor type of processes;

however, there are physical differences that must be

understood and properly evaluated for effective

design. The state-of-the-art for design of these fixed-

film processes has also been significantly advanced

over the past few years. Initial design concepts

1970 EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste

considered hydraulic flow rate or waste water organic

concentration. More advanced approaches consider

soluble substrate removal per unit surface area or

contact time based on the assumption of first-

or second-order reaction rate kinetics throughout

the system. Still other approaches consider the rela-

tionship of treatment efficiency versus total organic

loading over a wide organic loading range which

allows determination of the proper design, consider-

ing variability in waste water characteristics as well as

effluent quality requirements. Based on these kinetic

design approaches, performance data, operating

information, and kinetic design data can be obtained

from bench and pilot-scale fixed-film systems for

design of full-scale systems in a manner similar to

activated sludge systems. These secondary biological

kinetic design approaches advance the state of the

art for consideration of the microbiological and bio-

chemical aspects of biological treatment, as well as

consideration of the engineered or operational con-

trol variables.

Scale-up Design Considerations

0020 Variations in waste water characteristics, environ-

mental conditions, and biological system performance

must be evaluated to design properly a biological

waste water treatment system to achieve the allowed

effluent criteria reliably. Variability in waste water

characteristics (flows and pollutant concentrations)

and variability in biological treatment performance

are two of the most important considerations during

scale-up and design of food-processing waste water

treatment facilities. It is extremely important during

the experimental planning to develop waste water

survey and biological treatability programs for col-

lection of sufficient data to evaluate the variability

and statistically analyze the data for design. The

waste water characterization requires knowledge of

the discharge characteristics due to production sched-

ules and clean-up operations and development of

sufficient characterization data for determining the

design conditions, for example at the 50 and 90%

probable flows and loadings. Consideration of the

data variability from the biological treatability study

due to the dynamic nature of the heterogeneous mi-

crobial populations is more complicated and in most

cases has not been properly addressed during design

of food-processing waste water treatment facilities.

0021 Modern concepts used for designing biological

waste water treatment processes for food-processing

waste waters employ mathematical models that

describe the relationships governing microbial

growth and substrate utilization (aerobic or anaer-

obic processes). These modeling approaches require

the development of biokinetic descriptive constants

that can be developed from treatability studies. Stover

and Kincannon have recently developed a mathemat-

ical model for both fixed-film and suspended-growth

aerobic and anaerobic biological systems that elimin-

ates the problem of variability in the biological

descriptive substrate utilization constants. The prob-

lems with the scatter observed in the development

of biokinetic coefficients utilized in other mathematic

models have been eliminated by expressing the mass-

substrate utilization rate as a function of the

mass substrate loading rates. The specific substrate

utilization rate (U) is plotted as a function of the

food-to-microorganism (F/M) ratio in activated

sludge systems and as a function of the mass loading

rate in kg day

1

1000 m

2

in fixed-film systems in

order to determine the biokinetic constants of U

max

and K

. These constants are then utilized in the math-

ematical model, and the remaining variability in

influent flow rate and influent substrate concentra-

tions can be handled effectively.

Fixed-film Reactor Design

0022In the Stover–Kincannon model for fixed-film react-

ors a substrate material balance is conducted around

the fixed-film system. The specific parameters of con-

cern for concept design and operational process

control for a biotower fixed-film process are shown

in Figure 1, and the definition of each of these param-

eters is presented in Table 1. When considering a

fixed-film reactor volume, a mass balance of sub-

strate into and out of that reactor volume can be

made as follows:

Mass of

substrate

into the

reactor

Mass of

substrate

out of the

reactor

Mass of

substrate

consumed

biologically

0023In the case of the fixed-film reactor, the reactor

volume is expressed in terms of thousand cubic

meters of media volume. Based on this approach,

the mass balance of substrate into and out of the

fixed-film reactor volume can be expressed as

follows:

¼ FS

dt A



A ð1Þ

where: F ¼ flow rate (m

1

); S

¼ influent substrate

concentration (mg l

1

); S

¼ effluent substrate con-

centration (mg l

1

); A ¼ surface area of volume

(1000 m

2

); and

dt A



¼ specific substrate utiliza-

tion rate (kg 1000 m

2

day

1

EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste 1971

0024 The surface area represents the active mass of

microorganisms. By plugging the following substrate

utilization rate term developed by Stover and Kincan-

non,

dt A



max

ðFS

Þ=A

þðFS

Þ=A

ð2Þ

into eq (1), the resulting expression can be solved for

the required surface area of media for design, as

follows:

A ¼

max

 S

 K

ð3Þ

where: U

max

¼ maximum specific substrate removal

rate (kg/1000m

day

1

); K

¼ proportionality

constant (kg/1000m

day

1

); and (FS

)/A ¼ specific

substrate loading rate (kg/1000 m

day

1

0025The expression can also be solved for the effluent

substrate concentration that would correspond to a

specific design loading and amount of media volume

available, as follows:

¼ S



max

þðFS

Þ=A

ð4Þ

0026Eqn (4) can then be used to evaluate system perform-

ance at existing facilities and project expected per-

formance for future increased loadings. Eqn (3) can

be used to determine design requirements to handle

additional loadings.

0027The biokinetic constants, U

max

and K

, can be

determined by linearizing eqn (2) or taking the

reciprocal as follows:

dt A



max



max

ð5Þ

where:

dt A



FðS

 S

ð6Þ

0028Now, the reciprocal of the specific substrate utiliza-

tion rate (F (S

 S

)/A) is plotted against the recipro-

cal of the specific substrate loading rate (FS

/A). In

this linearized form, the y-axis intercept is equal to 1/

max

, and the slope of the line is equal to K

max

Activated Sludge Reactor Design

0029In the Stover–Kincannon model for activated sludge

reactors, a substrate material balance is conducted

around the aeration basin and the secondary clarifier,

in a similar manner to that for the fixed-film reactor

Forward flow

Recirculation sump

Effluent

Biotower

F, S

F + R, S

F + R , S

Forward flow + Recirculation

fig0001 Figure 1 Basic process flow diagram for biotower system with operating parameters.

tbl0001 Table 1 Definition of biotower process parameters

F Forward flow rate (m

1

)

R Recirculation rate (m

1

)

F þ R Total flow to the biotower (m

1

)

Substrate concentration in the forward-flow stream

(mg l

1

)

Substrate concentration in the biotower effluent

(mg l

1

)

Substrate concentration in the actual biotower influent

(mg l

1

)

d Biotower diameter (m)

h Biotower media height (m)

Specific surface area of media (m

3

)

n Rotational speed of the distributor (rev min

1

)

N Number of arms in rotary distributor assembly

1972 EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste

model. The specific parameters of concern for con-

cept design and operational process control for

activated sludge systems are shown in Figure 2, and

the definition of each of these parameters is presented

in Table 2. When considering an aeration basin

volume, the mass balance of substrate into and out

of that reactor volume can be expressed as follows:

Mass of

substrate

into the

reactor

Mass of

substrate

out of the

reactor

Mass of

substrate

consumed

biologically

0030Based on this approach, when considering an acti-

vated sludge reactor volume, a mass balance of sub-

strate into and out of the activated sludge reactor

volume can be made as follows:

¼ FS



V ð7Þ

where: F ¼ influent flow rate (m

1

); S

¼ influent

substrate concentration (mg l

1

); S

¼ effluent sub-

strate concentration (mgl

1

); V ¼ reactor volume

); and



¼ specific substrate utilization rate

(kg kg

1

day

1

0031By plugging the following substrate utilization rate

term developed by Stover and Kincannon into eqn

(7), the resulting expression can be solved for the

required aeration basin volume for design, as follows:



max

XðFS

Þ=XV

þ FS

=XV

ð8Þ

V ¼

max

 S

 K

ð9Þ

where: U

max

¼ maximum specific removal rate

(kg kg

1

day

1

); K

¼ proportionality constant, or

substrate loading at which the rate of substrate util-

ization is one-half the maximum rate (kg kg

1

day

1

);

/XV or F/M ¼ specific substrate loading rate (kg

1

day

1

); and X ¼ biological solids concentration

(mg l

1

0032The expression can also be solved for the effluent

substrate concentration that would correspond to a

specific design loading, mixed liquor sludge concen-

tration, and amount of reactor volume available, as

follows:

¼ S



max

þ FS

=XV

ð10Þ

0033Eqn (10) can then be used to evaluate system per-

formance at existing facilities and project expected

performance for future increased loadings. Eqn (9)

can be used to determine design requirements to

handle additional loadings.

0034The biokinetic constants, U

max

and K

, can be

determined by linearizing eqn (8) or taking the recip-

rocal as follows:



max



max

ð11Þ

where:





FðS

 S

ð12Þ

tbl0002 Table 2 Definition of activated sludge parameters

F Influent flow rate (m

1

)

Influent substrate concentration (mg l

1

)

Influent solids concentration (mg l

1

)

Effluent substrate concentration (mg l

1

)

X Biological solids concentration, MLVSS

(mixed liquor volatile suspended solids) (mg l

1

)

V Aeration reactor volume (m

)

Effluent suspended solids concentration

(mg l

1

)

Recycle solids concentration (mg l

1

)

Waste sludge flow rate (m

1

)

a Recycle ratio

a F Recycle flow rate (m

 1

)

max

Maximum specific substrate utilization

rate (kg kg

1

day

1

)

Substrate loading rate at which the rate

of substrate utilization is one-half the

maximum rate (kg kg

1

day

1

)

True cell yield

Endogenous decay rate

obs

Observed sludge production yield

(kg kg

1

substrate removed)

F/M Food-to-microorganism ratio

(kg kg

 1

day

1

)

U Specific substrate utilization rate

(kg kg

 1

day

1

)

, F, X

X,S

, S

, X

, S

αF, X

, X

Bioreactor

(1+α)*F

Air

Clarifier

Return sludge

F − F

fig0002 Figure 2 Flow diagram of the activated sludge process show-

ing notation and mass balance envelope.

EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste 1973

0035 Now, the reciprocal of the specific substrate utiliza-

tion rate (F (S

 S

)/XV) is plotted against the recip-

rocal of the specific substrate loading rate (FS

/ XV ¼

F/M). In this linearized form, the y-axis intercept is

equal to 1/U

max

, and the slope of the line is equal to

max

. Once the biokinetic constants have been

determined, the required F/M ratio at any given

loading (F and S

) can be determined to meet the

required effluent quality reliably (S

). The biokinetic

constants, which represent the treatability character-

istics of the waste water, can be determined experi-

mentally for any given activated sludge process.

These constants can be developed from pilot plant

or laboratory-scale studies, as well as, from the full-

scale plant data.

Aerobic Thermophilic Process Design

Considerations

0036 Evaluations, both bench-scale and pilot-scale operat-

ing systems, of aerobic thermophilic reactors have

shown that these systems comply with the same

types of monomolecular kinetic relationships previ-

ously described. Resultant data analyses have shown

that the relationships of substrate removal versus

substrate applied were applicable to aerobic thermo-

philic systems. This mathematical model has been

used extensively in the design and operation of

aerobic thermophilic treatment systems. It is import-

ant to note that the kinetics of microbial thermophilic

growth exceeds the growth of mesophiles and higher

substrate utilization rates increase the amount of sub-

strate that can be effectively treated in a given reactor.

0037 Primary design considerations for aerobic thermo-

philic biological treatment systems include the

following:

0038 temperature

0039 aeration and mixing

0040 matching oxygen transfer with oxygen demand

0041 sufficient biodegradable organics

0042 food-to-microorganism ratio

0043 solids retention time

0044 kinetics of substrate removal and biomass growth

0045 heat loss management

0046 foam control

0047 The thermophilic reactor and mixing and oxygen

transfer system are designed for minimal heat loss,

maximum mixing intensity, and maximum oxygen

transfer capacity in order to optimize and control

the reaction temperature. The reactor is designed to

minimize heat loss at a variable operating liquid

depth by minimizing the cross-sectional surface

area to volume ratio. Then with proper reactor

material of construction and insulation, the reactor

temperature can be controlled by varying the liquid

depth, the liquid mixing flow rate, and the air flow

rate.

0048When considering liquid waste water treatment,

the following factors are important relative to main-

taining autothermal conditions:

0049COD concentration/loading

0050(F/M) ratio

0051oxygen balance (COD balance)

005212 700–13 200 kJ kg

1

0053The organics or COD goes to new biomass and

carbon dioxide. The primary objective of thermo-

philic treatment is to convert the majority of the

organics to carbon dioxide and heat energy while

minimizing the biomass production. Nitrification is

also inhibited above about 45



C, such that excess

oxygen requirements are not observed for nitrifica-

tion. Excess ammonia-nitrogen reacts with the

carbon dioxide to produce ammonium bicarbonate

alkalinity to assist with reactor pH control.

0054When considering aerobic thermophilic treatment

(stabilization) of waste biological sludges the fol-

lowing equation provides valuable insight into the

reaction process:

N þ 5O

! 5CO

þ NH

þ Energy þ Heat

ð13Þ

0055By understanding this simple biochemical relation-

ship and the following factors, maintaining autother-

mal conditions and process control can be related to

the following factors:

0056oxygen (COD) balance (1.42 K

VS (volatile

solids) destroyed)

0057VS concentration/loading

0058VS destruction

0059VS balance

0060Nitrogen balance

006118 800–19 200 kJ kg

1

VS destroyed

Anaerobic Process Design

Considerations

0062In addition to controlling the loading rate, the meso-

philic anaerobic microbes require an optimum tem-

perature (30–35



C), a pH range of 6.7–7.2, adequate

nutrients, and no toxic organic or inorganic com-

pounds. Certain trace inorganic nutrients and vita-

mins are needed in anaerobic treatment processes.

Trace metals such as nickel, iron, cobalt, molyb-

denum, selenium, and tungsten are stimulatory to

methanogens at low concentrations. It is extremely

important to monitor and control these parameters

during start-up and operations of anaerobic reactors.

1974 EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste

In addition, adequate amounts of calcium and

magnesium are required to promote the growth

of sludge granules which are prevalent in the anaer-

obic sludge blanket and hybrid processes.

0063 The most effective indicators of performance are

pH, volatile acid to alkalinity ratio, and gas (me-

thane) production. The key to successful start-up is

to increase the biological solids inventory by gradual

increases in the flow and organic loading to the plant.

It is critical that the loadings are increased in propor-

tion to the biomass in order to control the F/M ratio.

This aspect of both plant start-up and stabilized plant

operations can be monitored by the system pH,

volatile fatty acids, volatile fatty acid/alkalinity

ratio, COD removal efficiency, methane content of

the gas, and reactor temperature. Adjustments are

required when any of these parameters exceed preset

acceptable operating ranges.

0064 Generally, the volatile acid/alkalinity ratio should

be maintained at less than 1.0 in order to obtain

optimum system performance. When this ratio

exceeds 1.0, immediate corrective action(s) should

be taken, otherwise the volatile acids will cause a

drop in the pH and the methanogen activity will

decrease. The methane production is a direct measure

of the metabolic activity of the methanogenic bacteria

and, as such, has great potential as a diagnostic tool

of anaerobic activity. The use of pH alone is not

as sensitive an indicator of reactor activity because

environmental changes will already have taken

place before a pH change is noticed. However, know-

ledge of the pH is important to good operation of the

system and should be maintained at approximately

6.7–7.2 for most applications.

Process Control Requirements

0065 In many cases in the past, biological treatment pro-

cesses for food industry wastes have been designed

and operated by general guidelines or rules of thumb.

However, with increasing treatment requirements, it

becomes necessary for treatment facilities to be

designed and operated more effectively to meet the

required effluent quality while continuing to treat

varying waste water flows and contaminant loads.

Operators must be better trained and educated so

that they are better equipped to monitor the bio-

logical system and make the proper corrections for

changing conditions. The treatment facility to be op-

erated must be designed with the greatest amount of

flexibility consistent with the design conditions for

that particular food-processing plant. This requires

the design engineer to rely on experience and oper-

ational feedback from full-scale operations, as well

as on parameters and observations obtained from

laboratory and pilot plant operations. After trans-

forming this operational experience into a treatment

plant design, the design engineer should be respon-

sible for proper training of the operators and for

assisting in the facility’s start-up.

0066Changing environmental conditions, especially

fluctuations in the waste water characteristics, tend

to disrupt the steady-state conditions that the bio-

logical treatment facilities are designed to approach.

Environmental changes tending to disrupt steady-

state conditions (shock loads) which cannot be, or

have not been, smoothed by preventive engineering

expedients must be accommodated solely by success-

ful biological response or by combined biological and

engineering remedial responses. To cope with these

situations successfully requires thorough understand-

ing of the process by the plant operational personnel

and the incorporation of adequate operating flexibil-

ity into the system by the design engineer.

0067The key to maintaining operational process control

and stable operations in biological treatment systems

is to provide proper environmental conditions to the

biomass or bacteria in the system. The hydraulic flow

rate and organic loading rate, along with the variabil-

ity in these parameters, are two of the most critical

parameters relative to maintaining stable operating

conditions; of course, pH, temperature, nutrients,

dissolved oxygen, and lack of toxic or inhibitory

substances are also critical to successful operations.

When properly monitoring and controlling these par-

ameters, the key to successful operations of activated

sludge systems then becomes matching the number of

microorganisms in the system to the organic substrate

loading rate to the system, or controlling the F/M

ratio of the system.

Operational Process Considerations

0068Kinetic approaches to successfully applying the

required metabolic and biological principles of food-

processing waste water treatment for both design and

operations have been discussed herein. The kinetic

constants U

max

and K

define the biological treatabil-

ity characteristics of the waste water. These biokinetic

constants are normally defined in terms of COD,

BOD, and/or total organic carbon (TOC) for food-

processing waste waters. They are used for design

purposes and operational process control and deci-

sion making when properly defined. They provide

significant operations value by denoting changing

qualitative waste water characteristics, toxicity inci-

dents, and refractory or residual nonbiodegradable

organic substances. For high-strength carbohydrate

waste waters without toxicity or refractory organics,

max

and K

are very close to one another in terms of

EFFLUENTS FROM FOOD PROCESSING/On-Site Processing of Waste 1975

BOD, COD, and TOC with values typically above

15 kg kg

1

day

1

in terms of COD. High residual/

refractory organics or toxicity will affect both the y-

axis intercept (U

max

) and the curvature (K

) of the

specific substrate utilization rate (U) versus the spe-

cific substrate loading rate (F/M ratio). Both refrac-

tory organics and toxicity reduce U

max

, and change

significantly from U

max

numerically. Therefore,

proper monitoring and use of these kinetic constants

can provide an invaluable tool for changing water

quality and maintaining stable operational process

control of biological systems treating food-processing

waste waters.

See also: Effluents from Food Processing: Microbiology

of Treatment Processes