Anaerobic Treatment of Dairy

www.elsevier.com/locate/procbio

Process Biochemistry 40 (2005) 2583–2595

Anaerobic treatment of dairy wastewaters: a review

Burak Demirel, Orhan Yenigun *, Turgut T. Onay

Institute of Environmental Sciences, Bogazici University, Bebek, 34342 Istanbul, Turkey

Received 11 May 2004; accepted 3 December 2004

Abstract

Anaerobic treatment is often reported to be an effective method for treating dairy effluents. The objective of this paper is to summarize

recent research efforts and case studies in anaerobic treatment of dairy wastewaters. The main characteristics of industrial dairy waste streams

are identified and the anaerobic degradation mechanisms of the primary constituents in dairy wastewaters, namely carbohydrates (mainly

lactose), proteins and lipids are described. Primary attention is then focused on bench–pilot–full-scale anaerobic treatment efforts for dairy

waste effluents. Combined (anaerobic–aerobic) treatment methods are also discussed. Finally, areas where further research and attention are

required are identified.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic treatment; Dairy wastewaters; Acidogenesis; Lipids degradation; Proteins degradation

1. Introduction

The dairy industry, like most other agro-industries,

generates strong wastewaters characterized by high biolo-

gical oxygen demand (BOD) and chemical oxygen demand

(COD) concentrations representing their high organic

content [1]. Furthermore, the dairy industry is one of the

largest sources of industrial effluents in Europe. A typical

European dairy generates approximately 500 m3 of waste

effluent daily [2]. Dairy waste effluents are concentrated in

nature, and the main contributors of organic load to these

effluents are carbohydrates, proteins and fats originating

from the milk [3,4]. Since dairy waste streams contain high

concentrations of organic matter, these effluents may cause

serious problems, in terms of organic load on the local

municipal sewage treatment systems [3]. In addition to

environmental problems that can result from discharge of

dairy wastewaters, introduction of products such as milk

solids into waste streams also represents a loss of valuable

product for the dairy facilities [5]. Most of the wastewater

volume generated in the dairy industry results from cleaning

of transport lines and equipment between production cycles,

cleaning of tank trucks, washing of milk silos and equipment

* Corresponding author. Tel.: +90 212 3596946; fax: +90 212 2575033.

E-mail address: [email protected] (O. Yenigun).

0032-9592/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2004.12.015

malfunctions or operational errors [4,6]. Dairy wastewaters

are treated using physico-chemical and biological treatment

methods. However, since the reagent costs are high and the

soluble COD removal is poor in physical–chemical

treatment processes, biological processes are usually

preferred [7]. Among biological treatment processes,

treatment in ponds, activated sludge plants and anaerobic

treatment are commonly employed for dairy wastewater

treatment [8]. In contrast the contrary, high energy

requirements of aerobic treatment plants is a significant

drawback of these processes. COD concentrations of dairy

effluents vary significantly; moreover, dairy effluents are

warm and strong, enabling them ideal for anaerobic

treatment [2]. Furthermore, no requirement for aeration,

low amount of excess sludge production and low area

demand are additional advantages of anaerobic treatment

processes, in comparison to aerobic processes. The aim of

this paper is to summarize the recent research efforts and

case studies in anaerobic treatment of dairy waste effluents.

In the paper, the general characteristics of dairy waste

streams are identified and the anaerobic degradation

mechanisms of the main constituents of dairy wastewaters,

namely carbohydrates, proteins and lipids, are explained.

Anaerobic treatment practices of dairy wastewaters, as

bench, pilot and full-scale efforts, are subsequently

introduced overall in detail. Combined (anaerobic–aerobic)

B. Demirel et al. / Process Biochemistry 40 (2005) 2583–25952584

treatment systems for dairy wastewaters are also summar-

ized briefly. Finally, areas where particular research and

more attention required in the near future are identified.

2. General characteristics of dairy wastewaters

Wastewaters from the dairy industry are usually

generated in an intermittent way, so the flow rates of these

effluents change significantly. High seasonal variations are

also encountered frequently and correlate with the volume of

milk received for processing; which is typically high in

summer and low in winter months [9]. Moreover, since the

dairy industry produces different products, such as milk,

butter, yoghurt, ice-cream, various types of desserts and

cheese, the characteristics of these effluents also vary

greatly, depending on the type of system and the methods of

operation used [7]. The use of acid and alkaline cleaners and

sanitizers in the dairy industry additionally influences

wastewater characteristics and typically results in a highly

variable pH [4,6,10]. Actually, information about the general

characteristics of dairy wastewaters from full-scale opera-

tions in literature is scarce. Only one comprehensive study

has been encountered, which provides extensive information

Table 1

Characteristics of dairy waste effluents

Effluent

type

COD

(mg/l)

BOD5

(mg/l)

pH

(units)

Alkalinity

(mg CaCO3/l)

Susp

solid

(mg/

Creamery 2000–6000 1200–4000 8–11 150–300 350–

Not given 980–7500 680–4500

Mixed dairy

processing

1150–9200 6–11 320–970 340–

Cheese whey 68814a

Cheese 1000–7500 588–5000 5.5–9.5 500–

Fresh milk 4656a 6.92a

Cheese 5340a 5.22a

Milk powder/

butter

1908a 5.80a

Mixed dairy

processing

63100a 3.35a 12

Cheese whey 61000a 1

Cheese 4.7a 2

Not given 4.4–9.4 90

Fluid milk 950–2400 500–1300 5.0–9.5 90

a Mean concentrations are reported.

Table 2

Concentrations of selected elements in dairy wastewaters

Effluent type Na (mg/l) K (mg/l) Ca (mg/l) Mg (mg/l)

Creamery 170–200 35–40 35–40 5–8

Cheese/whey 735a 42.8a 47.7a 11.4a

Cheese/alcohol 423a 41.2a 54.3a 8.3a

Cheese/beverages 453a 8.6a 33.6a 16.9a

Cheese/whey 419a 35.8a 52.3a 11.0a

Mixed dairy 123–2324 8–160 12–120 2–97

Cheese 720–980 530–950

a Mean concentrations are reported.

about the particular characteristics of dairy wastewaters

from various full-scale operations [6]. A summary of data

obtained from literature for general properties of dairy waste

effluents from full-scale operations is given in Table 1 [11–

19]. High COD concentrations indicate that dairy industry

wastewaters are strong and fluctuating in nature. Significant

fractions of the organic components and nutrients in dairy

waste streams are derived from milk and milk products. In

industrial dairy wastewaters, nitrogen originates mainly

from milk proteins, and is present in various forms; either an

organic nitrogen (proteins, urea, nucleic acids), or as ions

such as NH4+, NO2

� and NO3�. Phosphorus is found mainly

in inorganic forms; as orthophosphate (PO43�) and poly-

phosphate (P2O74�), as well as organic forms [20]. Concen-

trations of suspended solids (SS) and volatile suspended

solids (VSS) are also used to evaluate wastewater strength

and treatability [6]. Suspended solids in dairy wastewaters

originate from coagulated milk, cheese curd fines or

flavoring ingredients [21]. Concentrations of selected

elements, namely sodium (Na), potassium (K), calcium

(Ca), magnesium (Mg), iron (Fe), cobalt (Co), nickel (Ni)

and manganese (Mn), are also given in Table 2. Particularly

high Na concentrations point out the use of large amount of

alkaline cleaners at dairy plants. The concentrations of

ended

s

l)

Volatile

suspended

solids (mg/l)

Total

solids

(mg/l)

TKN

(mg/l)

Total

phosphorus

(mg/l)

Reference

1000 330–940 50–60 [4]

300 [9]

1730 255–830 2705–3715 14–272 8–68 [11]

1462a 379a [12]

2500 [13]

[14]

[14]

[14]

500a 12100a 53000a [15]

780a 1560a 980a 510a [16]

500a 830a 280a [17]

–450 [18]

–450 [19]

Fe (mg/l) Co (mg/l) Ni (mg/l) Mn (mg/l) Reference

2–5 0.05–0.15 0.5–1.0 0.02–0.10 [4]

[6]

[6]

[6]

[6]

0.5–6.7 0 0–0.13 0.03–0.43 [11]

[13]

B. Demirel et al. / Process Biochemistry 40 (2005) 2583–2595 2585

heavy metals, such as copper (Cu), nickel (Ni) and zinc (Zn)

were reported to be in a range that would not affect adversely

the performance of a biological treatment step [6,11].

As stated above, dairy wastewater is composed of easily

degradable carbohydrates, mainly lactose, as well as less

biodegradable proteins and lipids [22]. In cheese-processing

wastewater, 97.7% of total COD was accounted for by lactose,

lactate, protein and fat [15]. Thus, dairy wastewater can easily

be defined as a complex type of substrate [22–24]. Lactose is

the main carbohydrate in dairy wastewater and is a readily

available substrate for anaerobic bacteria. Anaerobic metha-

nation of lactose needs a cooperative biological activity from

acidogens, acetogens and methanogens [25]. Anaerobic

fermentation of lactose yields organic acids, namely acetate,

propionate, iso- and normal-butyrate, iso- and normal

valerate, caproate, lactate, formate and ethanol [26,27].

Two possible carbon flow schemes were proposed for

acidogenic fermentation of lactose; carbon flow from

pyruvate to butyrate and lactate, both occurring in parallel

[28]. The presence of high carbohydrate concentrations in

synthetic dairy wastewater was found to reduce the amount of

proteolytic enzymes synthesized, resulting in low levels of

protein degradation [22]. It was previously reported that

carbohydrates could suppress the synthesis of exopeptidases,

a group of enzymes facilitating protein hydrolysis [29].

Anaerobic degradation of proteins and the effects of ammonia

on this mechanism were recently investigated in detail [30–

35]. Casein is the major protein in milk composition and in

dairy effluents. When fed to acclimated anaerobic reactors,

degradation of casein is very fast and the degradation products

are non-inhibitory [3].

Lipids are potentially inhibitory compounds, which can

always be encountered during anaerobic treatment of dairy

wastewaters. There is little information available in literature

about the anaerobic digestibility of lipids. During anaerobic

degradation, lipid is firstly hydrolyzed to glycerol and long

chain fatty acids (LCFAs), followed by b-oxidation,

producing acetate and hydrogen [29]. The biodegradation

of lipids is difficult due to their low bioavailability [36].

Glycerol, a compound formed as a result of lipid hydrolysis,

was found to be a non-inhibitory compound [3], while,

LCFAs were particularly reported to be inhibitory to

methanogenic bacteria [37]. The inhibitory effects of lipids

in anaerobic processes can mainly be correlated to the

presence of LCFAs, which cause retardation in methane

production [38]. Lipids do not cause serious problems in

aerobic processes, however, they sometimes affect conven-

tional single-phase anaerobic treatment processes adversely

[39,40]. Unsaturated LCFAs seemed to have a greater

inhibitory effect than saturated LCFAs. Unsaturated LCFAs

strongly inhibited methane production from acetate and

moderately inhibited b-oxidation. Thus, unsaturated LCFAs

should be saturated to prevent lipid inhibition in anaerobic

processes [40]. Difficulties experienced with the presence of

lipids in anaerobic treatment processes have been previously

reported in literature[41–44].

3. Conventional (single-phase) anaerobic treatment

of dairy effluents

Anaerobic treatment processes are favourable methods

for treating dairy waste effluents, in comparison to aerobic

processes, due to their well-known benefits for treating

industrial wastewaters, particularly from agricultural indus-

tries with a high organic content [45–47]. Anaerobic

treatment applications for dairy industry wastewaters have

been evaluated in a number of previous studies [48–59].

More recent information about anaerobic treatment prac-

tices of dairy waste streams are also displayed in Table 3.

In treatment studies of dairy wastewaters, anaerobic filters

have recently been used. If the particular dairy effluents

contain low concentrations of suspended solids, then

anaerobic filter reactors are generally suitable for biological

treatment. A laboratory-scale plastic medium anaerobic filter

reactor provided average COD removal rates between 78 and

92%, at a hydraulic retention time (HRT) of 4 days [60,61]. In

addition, the start-up performance of the anaerobic filters

treating dairy wastewater, in terms of COD removal, was not

significantly affected by temperature variations between 21

and 30 8C [62]. The effects of porous and non-porous support

media in anaerobic filter reactors on thermophilic anaerobic

treatment of ice-cream wastes were investigated extensively

[63,64]. At high loading rates, anaerobic filter with porous

support media performed more satisfactorily [63]. The

performance of porous and non-porous media in an upflow

anaerobic filter (UAF) treating wastewaters from a milk

bottling factory was also investigated in a more recent work

[65]. The reactor with non-porous packing showed instability

above an organic loading rate (OLR) of 4 kg COD/(m3 day),

while the reactor with porous packing was still stable at OLRs

up to 21 kg COD/(m3 day). The results of a pilot trial showed

that the anaerobic filter reactor, treating wastewaters from an

ice-cream manufacturer, achieved a mean COD removal rate

of 70%, at an average load of 5.5 kg COD/(m3 day) [66]. Low

temperature kinetics of anaerobic filters treating dairy

wastewaters were determined for various HRT ranges, using

three reactors operated at 12.5, 21 and 30 8C [67]. A

relationship was developed between the temperature of the

system and the first-order rate constant for each anaerobic

filter reactor. In another work, an upflow anaerobic filter

reactor (UAF) was used to treat very dilute dairy wastewater,

in an OLR range of between 0.117 and 1.303 g volatile solids

(VS)/(l day) and at an HRT range between 18.8 and 2 days

[68]. At 5, 4 and 3 days of HRT, effluent SS and COD

concentrations satisfied the effluent discharge limits. A pilot-

scale upflow anaerobic filter reactor treating dairy waste-

waters provided more than 85% COD and 90% BOD removal,

at an OLR of 6 kg COD/(m3 day) [69]. At 20 h of HRT, the

percentage of methane in the biogas produced by the UFAF

was found to be in a range between 75 and 85%, with a

corresponding methane yield of about 0.33 m3 CH4/kg COD

removed. The system could generate approximately 770 l

methane (CH4)/day. A comparative study of staged and

B.

Dem

irelet

al./P

rocess

Bio

chem

istry4

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59

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58

6Table 3

Anaerobic treatment performance levels for dairy wastewaters

Waste type Reactor type HRT Loading Temperature

(8C)

Methane yield

(m3/kg CODra)

Removal (%) Application

status

Reference

Cheese whey Downflow fixed-film 4.9 (day) 13 (kg COD/(m3 day)) 0.28 75 (COD) Pilot scale [16]

Cheese whey Downflow fixed-film 6.6 (day) 8.3 (kg COD/(m3 day)) 0.34 76 (COD) Pilot scale [16]

Anaerobic filter 4 (day) 30–21–12.5 92–85–78 (COD) Laboratory scale [60,61]

Ice-cream Anaerobic filter 5.5 (kg COD/(m3 day)) 75 (COD) Pilot scale [66]

Anaerobic filter 6 (kg COD/(m3 day)) 0.32–0.34 85 (COD) Pilot scale [69]

Anaerobic filter 0.5 (day) Up to 21 (kg COD/(m3 day)) 80 (COD) [72]

Raw milk Anaerobic filter 5–6 (kg COD/(m3 day)) 90 (COD) Full scale [73]

Cheese whey UASB >97 (COD) Laboratory scale [75]

Cheese production

wastewater

UASB 31 (g COD/(l day)) 90 (COD) Laboratory scale [78]

Whey permeate UASB 0.4–5 (day) 99–64.2 (COD) Laboratory scale [80]

Effluent of an

integrated plant

Hybrid UASB 5 (h) 8.5 (g COD/(l day)) 30 87 (COD) Laboratory scale [19]

Hybrid UASB 1–8 (g COD/(l day)) 30 92 Laboratory scale [82]

Cheese production

wastewater

6 2–7.3 85–99

10–20 2.3–4.5 35 79–91 Laboratory scale [17]

UASB 30–40 (day) 1.5–1.9 (g COD/(l day)) 81

UASB 90 (COD) [83]

Synthetic dairy

effluent

Hybrid 4.1–1.7 (day) 0.82–6.11 (kg COD/(m3 day)) 35 0.354 (at 1.7

days HRT)

90–97 (COD) Laboratory scale [85]

Cheese whey Hybrid 2 (day) Up to 11 (kg COD/(m3 day)) >95 (COD) Laboratory scale [86]

Synthetic non-fat

dry milk

ASBR 6 (h) 5 62 (COD), 75 (BOD5) Laboratory scale [87]

ASBR 3–6 (day) 2–4 (g VS/(l day)) 26–50 (VS) Laboratory scale [88,89]

Upflow packed-bed 5–14.29 (kg COD/(m3 day)) 93.8–98.5, 72.5–84 (COD) [92]

Washing and

rinsing waters

90.4 (COD) Laboratory scale [93]

Cheese whey Rotating biological

contact reactor

3 (day) 37 85 (COD) [95]

Upflow anaerobic solid

removal reactor

4.5 (h) 20 98 (lipid) Laboratory scale [96]

Cheese whey Multichamber bioreactor 2 (day) 37 83 (COD) [98]

Synthetic ice-cream

wastewater

CSTR 7.45–5.99–4.60

–3.76–2.99 (day)

35 98–97–96–94–92 (SCOD)b Laboratory scale [100]

a COD removal.b Soluble COD.


non-staged anaerobic filters treating synthetic dairy waste-

waters was conducted at an HRT of 2 days and in a substrate

concentration range of from 3 to 12 g COD/l [70]. The authors

reported that the performances of both reactors were very

similar under the operating conditions tested. Laboratory-

scale single-fed (SFR) and multi-fed (MFR) upflow anaerobic

filters treating cheese whey were operated at OLRs above

20 kg COD/(m3 day) [71]. It was observed that the feeding

regime affected both biomass concentration and activity. The

specific activities of the different trophic groups were found to

be higher in the MFR. An upflow anaerobic filter reactor

yielded an average of 80% COD removal in an OLR range up

to 21 kg COD/(m3 day) when treating dairy wastewater [72].

Recently, the performance of an industrial-scale anaerobic

filter treating raw milk discharged by quality control

laboratories was reported [73]. Higher than 90% of COD

removal could be attained, with an OLR maintained around 5–

6 kg COD/(m3 day). Moreover, the fat content in dairy

wastewater could successfully be degraded by the anaerobic

filter reactor.

Upflow anaerobic sludge blanket (UASB) reactors have

been successfully employed for dairy wastewater treatment

in full-scale applications for almost two decades [74].

Biological treatment of a cheese-producing wastewater by a

laboratory-scale UASB reactor was reported for influent

wastewater concentrations between 12 and 60 g COD/l [17].

COD removal rates varied between 85 and 99%, at an HRT

of 6 days and in an OLR range of from 2 to 7.3 g COD/

(l day), while removal rates were around 81% in an HRT

range between 30 and 40 days. Anaerobic treatability studies

of dairy effluents from a large integrated industry processing

milk were carried out using a laboratory-scale hybrid UASB

reactor [19]. At an OLR of 8.5 g COD/(l day) and an HRT of

5 h, 87% COD removal was achieved at 30 8C. Another

laboratory-scale investigation pointed out that more than

97% COD reduction could be achieved in an UASB reactor

during anaerobic treatment of cheese whey [75]. Methanol

addition during start-up of UASB reactors treating a dairy

waste from raw ice-cream production facility provided rapid

granulation of biomass and enhanced the settling velocity

and specific activity of the sludge [76]. However, methanol

addition also resulted in severe biomass wash-out from the

system. During laboratory-scale anaerobic digestion of

cheese whey, increased substrate loadings led to the failure

of the UASB reactor, in a range of influent substrate

concentration from 4.5 to 38.1 g COD/l, at an HRT of 5 days

[77]. Anaerobic treatment of cheese-production wastewater

using a laboratory-scale UASB reactor provided a COD

removal of about 90%, at an OLR of 31 g COD/(l day) [78].

Furthermore, OLR peaks above 45 g COD/(l day) yielded a

COD reduction between 70 and 80%. Sudden increase in the

OLR was accompanied by biomass granulation, resulting in

a more stable reactor operation during waste treatment. The

kinetic model and the kinetic coefficients of laboratory-scale

continuous UASB reactors treating whey permeate were

determined, in an HRT range between 0.4 and 5 days and at a

constant influent substrate concentration of 10.4 � 0.2 g

COD/l [79]. The maximum substrate utilization rate (k), half

saturation coefficient (KL), yield coefficient (Y) and decay

rate coefficient (Kd) were determined to be 0.941,

0.773 kg COD/(kg VSS day), 0.153 kg VSS/(kg COD) and

0.022 day�1, respectively. Under the same HRT and influent

substrate concentration, COD removal efficiency in the

UASB reactor ranged between 64.2 and 99% [80]. The

anaerobic digestion of cheese whey was investigated, in

terms of instability caused by the strength of the influent in

an UASB reactor [81]. For proper system stability, the

optimum influent substrate concentration was determined to

be between 25 and 30 g COD/l at an HRT of 5 days. A

laboratory-scale combined system designed by converting

the flow mixing chamber of an anaerobic filter into an UASB

resulted in an organic matter removal of 92% for dairy

wastewater, in an OLR range of between 1 and 8 g COD/

(l day), at 30 8C [82]. Dairy wastewaters containing high

concentrations of fat and grease were treated by an UASB

reactor [83]. COD removal was reported to be about 90%.

Furthermore, a new generation of more advanced anaerobic

reactor systems have also been developed, based on the

UASB system. A successful version of this concept is the

internal circulation (IC) reactor [84]. The IC reactor system

is able to handle high upflow liquid and gas velocities, which

enables treatment of low strength wastes at short HRTs, as

well as treatment of high-strength effluents at very high

volumetric loading rates feasible. Recently, feasibility of

using UASB reactors for dairy wastewater treatment was

explored by operating two types of UASB reactors [85]. The

reactors were operated at an HRT range between 3 and 12 h,

and on loadings ranging from 2.4 to 13.5 kg COD/(m3 day).

At 3 h, maximum COD reduction ranged between 95.6 and

96.3%, while at 12 HRT reductions were around 92–90%,

for both reactors.

In addition to anaerobic filters and UASB reactors, hybrid

digesters and anaerobic sequencing batch reactors (ASBR)

are also employed for treating dairy effluents. A mesophilic

laboratory-scale hybrid anaerobic digester, combining an

upflow sludge blanket and a fixed-bed design, was used to

treat synthetic dairy effluent with an influent substrate

concentration of 10 g COD/l [86]. At an HRT range between

4.1 and 1.7 days, COD removal rates between 90 and 97%

could be achieved, at an OLR range between 0.82 and

6.11 kg COD/(m3 day). The anaerobic digester provided a

methane yield of 0.354 m3 CH4/kg COD removed at an HRT

of 1.7 days. Anaerobic treatment of a high-strength acidic

cheese whey by a laboratory-scale upflow hybrid reactor

resulted in removal efficiencies of more than 95%, at 2 days

of HRT and up to an OLR of about 11 kg COD/(m3 day)

[87]. ASBRs also provide high treatment efficiencies for

dairy effluents. The laboratory-scale ASBR system was

reported to provide soluble COD and BOD5 removal rates of

62 and 75%, respectively, at an HRT of 6 h, at 5 8C, for a

synthetic substrate of non-fat dry milk [88]. In a temperature

range between 5 and 20 8C, and at an HRT range between 24


and 6 h, soluble organic removal rates ranged between 62

and 90% for COD, and 75 and 90% for BOD5. In another

laboratory-scale work, two-stage thermophilic ASBR

systems provided volatile solids removal of 26–44%, while

mesophilic ASBR systems achieved VS removal between 26

and 50% for dairy wastewater [89,90]. The systems were

operated in an OLR range of 2–4 g VS/(l day) and at HRTs

of 3 and 6 days. The purification performance and the basic

fundamentals for the design of an ASBR used for treating

concentrated dairy wastewater were investigated in a more

recent study [91]. The maximum loading rate was

determined to be 6 g COD/(l day) for stable operation,

since higher loadings were reported to cause problems, such

as sludge removal and purification efficiency declines.

Actually, more studies focusing on different aspects of

anaerobic treatment of dairy wastewaters were performed

during the last decade. The changes in the microbial ecology

in four different pilot-scale digesters, namely anaerobic

contact, anaerobic filter, anaerobic expanded/fluidized bed

reactor and UASB reactor, all treating ice-cream wastewater,

were comprehensively examined during start-up [92]. The

authors reported that the reactor configuration did not play

an important role in causing changes in the microbial

community. A two-stage upflow packed-bed reactor system

was employed to treat dairy wastewater [93]. The maximum

COD removal rates obtained were between 93.8 and 98.5,

and 72.5 and 84%, respectively, for the two reactors

operated. The kinetic constants of anaerobic digestion of

dairy industry wastewater were determined using bioreac-

tors containing suspensions of micronized clays [94]. The

yield coefficient of methane (Yp) was computed to be

342 ml CH4/g COD. The system achieved around 90% COD

removal on average.

In another study, it was observed that bioconversion of

whey generated from cheese and casein manufacturing

process resulted in a pollutant reduction greater than 75%

[95]. The anaerobic treatment of salty cheese whey was

investigated using an anaerobic rotating biological contact

reactor [96]. The optimum performance was attained at an

HRT of 3 days and at 37 8C, resulting in a COD reduction of

85%. The methane content of the reactor biogas was around

74%. Application of an upflow anaerobic solid removal

(UASR) reactor was evaluated during pre-treatment of a

dairy wastewater [97]. At 20 8C and at 4.5 h of HRT, 98% of

lipids were removed at a pH of 4.0. A multichamber

anaerobic bioreactor was used to treat salty cheese whey

diluted with a mixed dairy wastewater [98]. The mesophilic

(37 8C) reactor provided a COD reduction of 83%, at an

HRT of 2 days. The methane content in the reactor biogas

was 68%. A non-attached bacterial growth process was

applied for biomethanation of a dairy wastewater [99]. In a

batch process, 92% COD reduction was obtained at 66 days.

The impacts of biofilm support systems on anaerobic

continuously stirred tank reactors (CSTRs) were described,

using a laboratory-scale reactor treating dairy wastewater

[100]. The authors reported that the incorporation of a

biofilm support system can significantly improve the

performance of the digester, due to development of active

biofilms that enhance biomass and waste contact, and reduce

the microbial wash-out. The process kinetics of the

mesophilic (37 8C) anaerobic digestion of synthetic ice-

cream wastewater were investigated at an HRT range

between 2.99 and 7.45 days, using the Monod and the

Contois equations, in a laboratory-scale study [101]. Since

the Contois equation incorporated the effect of any changes

in the influent substrate concentration, this kinetic model

described the process kinetic coefficients of the pilot-scale

anaerobic contact process better.

4. Two-phase anaerobic treatment of dairy wastewater

Two-phase anaerobic treatment systems are particularly

suitable for wastewaters containing high concentrations of

organic suspended solids, such as food and agricultural

industry wastewaters [102,103]. The performance of an acid

phase (acidogenic) reactor is especially of paramount

importance during two-phase anaerobic stabilization of

wastes, since the acid reactor should provide the most

appropriate substrate for the subsequent methane phase

(methanogenic) reactor. The guidelines for the design of pre-

acidification reactors for treating various high-strength

industrial wastewaters were already presented in literature

[104]. Actually, numerous studies have been performed

covering particularly the anaerobic acidogenesis of dairy

wastewaters. Initially, acidogenesis of lactose was investi-

gated extensively in laboratory-scale studies, mostly

focusing on the degradation kinetics of lactose [25–

28,105]. Studies covering the acid phase digestion of

industrial and synthetic dairy wastewaters have been

reviewed in different aspects previously in this paper

[10,11,14,15,22,24]. In one of these studies, anaerobic

digestion of three different dairy effluents (namely cheese,

fresh milk and milk powder/butter factories) was evaluated,

using a laboratory-scale mesophilic two-phase system [14].

For the cheese factory effluent, 97% COD removal was

achieved at an OLR of 2.82 kg COD/(m3 day), while at an

OLR of 2.44 kg COD/(m3 day), 94% COD removal was

obtained for the fresh milk effluent. For the powder milk/

butter factory effluent, 91% COD removal was achieved at

an OLR of 0.97 kg COD/(m3 day). In addition to these

works, the operating criteria for pre-acidification of dairy

wastewater obtained from a milk and cream bottling plant

were determined in a laboratory-scale CSTR [106]. The

maximum acidogenic conversion was computed to be 71%

by the authors. Acetic, propionic, n-butyric and n-valeric

acids were commonly produced during acidogenesis of dairy

wastewater. Anaerobic production of volatile fatty acids

(VFAs) with fermentation of whey permeate (wastewaters

from cheese-making process) was investigated in a

laboratory-scale anaerobic fluidized bed reactor [107]. It

has been shown that up to 19% of the initial sugar was


converted to volatile acids. Production of n-butyric acid was

also favoured during whey biodegradation. The influence of

substrate strength on thermophilic anaerobic acidogenesis of

a simulated dairy wastewater was investigated in a

laboratory-scale work [108]. In an influent substrate

concentration range between 2 and 30 g COD/l and at

55 8C, carbohydrate degraded under all conditions, however,

protein and lipid conversions both decreased when the

substrate concentration increased. The major acidogenesis

products were measured to be acetate, propionate, butyrate

and ethanol. Acidification of mid- and high-strength

synthetic dairy wastewaters were studied in a laboratory-

scale upflow reactor [109]. The authors reported that the

high-strength wastewater favoured production of hydrogen

(H2) and alcohols. Inhibitory effects of zinc (Zn) and copper

(Cu) on anaerobic acidogenesis of dairy wastewater were

investigated in a laboratory-scale study [110]. The authors

concluded that copper seemed more toxic than zinc to the

overall production of VFAs and hydrogen in the acidogenic

reactor. Start-up of two acidogenic reactors under meso-

philic (37 8C) and thermophilic (55 8C) conditions were

compared using a methanogenic granular sludge and dairy

wastewater [111]. It took more than 2 months to establish a

microbial community with a stable metabolic activity.

Acidogenesis of synthetic dairy wastewater was also studied

at a pH range between 4.0 and 6.5, in a laboratory-scale

upflow reactor at 37 8C [112]. At an HRT of 12 h and a pH of

5.5, 95% of carbohydrates, 82% of proteins and 41% of

lipids could be degraded in the acid phase reactor. Moreover,

batch reactors were used to investigate the thermophilic

anaerobic acidification of a synthetic dairy wastewater at a

pH of 5.5 [113]. According to the authors, hydrogen

production could be attributed to the fermentation of

carbohydrate. Two acidogenic upflow reactors were

operated under mesophilic (37 8C) and thermophilic

(55 8C) conditions with a synthetic dairy wastewater, in

order to compare the effects of temperature on the

Table 4

Two-phase anaerobic treatment process performances for dairy wastewater treatm

Waste type Reactor type HRT Loading

Cheese-fresh milk-powder

milk/butter effluents

2.82–2.44–0.97

(kg COD/(m3 day))

Dilute milk waste CSTR + upflow

filter

4.4 (day)

Fluidized beds 9.4 (kg COD/

(m3 day))

Wastewater of a milk

bottling plant

CSTR + upflow

filter

2 (day)

Skimmed milk CSTR + upflow

filter

2 (day)

Wastewater of a milk and

cream bottling plant

CSTR + upflow

filter

2 (day) 5 (kg COD/

(m3 day))

Synthetic cheese whey CSTR + upflow

filter

5 (day)

performances of both reactors [114]. Almost no difference

was reported for the performances of both reactors in terms

of COD removal and degree of acidification, at any given

OLR. A cheese-processing wastewater was used to

determine the biokinetics of mesophilic acidogens [115].

At a pH of 7 and 36.2 8C, the maximum microbial growth

rate (mmax), half saturation coefficient (Ks), the microbial

yield coefficient (Y) and microbial decay rate (kd) were

computed to be 9.9 day�1, 134 mg COD/l, 0.29 mg MVSS/

mg COD and 0.14 day�1, respectively. While at a pH of 7.3

and 36.2 8C, mmax, Ks, Y and kd were determined to be

9.3 day�1, 482.5 mg COD/l, 0.20 mg MVSS/mg COD and

0.25 day�1, respectively. In a more recent study, the effects

of HRT between 12 and 24 h on anaerobic acidogenesis of

dairy wastewater was investigated, using a laboratory-scale

continuous-flow completely mixed anaerobic reactor with

solids recycle [116]. The acid production gradually

increased proportionally to the OLR, with decrease in

HRT. The highest degree of acidification and the rate of acid

production were 56% and 3.1 g/(l day), respectively, at 12 h

of HRT.

A summary of data for two-phase anaerobic dairy waste-

water treatment practices in literature is given in Table 4.

Initial results for two-phase anaerobic treatment of a dairy

wastewater were reported from pilot plant studies [117]. The

authors reported that the fluidized bed reactor configuration

was best suited to the two-phase operation, since this process

could achieve a high concentration of biomass in the reactor

without the need for the bacterial separation between the

stages. Two-phase anaerobic treatment of dilute milk wastes

was investigated, using a CSTR for acidogenesis and an

upflow filter reactor for methanogenesis, respectively [118].

The system attained 92% COD removal at an overall HRT of

4.4 days for an influent substrate concentration of

1500 mg COD/l. Anaerobic fluidized beds were also used

in another work, in order to treat a dairy effluent with an

influent COD concentration of 5000 mg COD/l [119].

ent

Temperature

(8C)

Methane yield

(m3/kg CODr)

Removal

(%)

Application

status

Reference

35 0.359–0.327

–0.287

97–94–91

(COD)

Laboratory

scale

[14]

92 (COD) [117]

76–92 (COD) [118]

35 90 (COD) Laboratory

scale

[120]

20 95.5 (COD) Laboratory

scale

[121]

33–36 90 (COD),

95 (BOD)

Laboratory

scale

[122]

35 95 (COD) Laboratory

scale

[123]


Substrate removal efficiencies varied between 76 and 92%, at

an OLR of 9.4 kg COD/(m3 day) in the two-phase system.

Anaerobic treatment of cheese whey was investigated in a

pilot-scale study [120]. The UASB reactors were used as

acidogenic and methanogenic reactors in this work. Two-

phase anaerobic digestion process for dairy wastewater

treatment was also examined from a microbiological point of

view during start-up in a laboratory-scale study [121]. The

authors reported that the numbers of acidogens remained

constant in the pre-acidification reactor (a CSTR), while the

numbers of methanogens and non-methanogens slightly

decreased in the methanogenic upflow anaerobic filter reactor.

Treatment characteristics of two different substrates, baby

nutrient milk and skimmed milk, were investigated using a

two-phase anaerobic digestion process at 20 8C [122]. At an

HRT of 2 days, 96% COD reduction was obtained for

skimmed milk with an influent substrate concentration of

1500 mg COD/l. The performance of a laboratory-scale

mesophilic two-phase anaerobic digestion system treating

dairy wastewater from a milk bottling plant was evaluated,

using a CSTR for acidogenesis and an upflow anaerobic filter

for methanogenesis [123]. The system achieved overall COD

and BOD removal rates of 90 and 95%, respectively, at an

HRTof 2 days and an OLR of 5 kg COD/(m3 day). In another

laboratory-scale study, a CSTR for acidogenesis and an

upflow anaerobic filter for methanogenesis were used at

Table 5

Maximum loadings and treatment rates in anaerobic treatment of dairy wastewa

Process mode Application status Wastewater type Reactor type

Single phase Pilot plant Milk and cream Upflow filter

Two phase Laboratory scale Cheese Hybrid

Single phase Laboratory scale Cheese UASB

Single phase Whey Semicontinuous

digester

Single phase Upflow filter

Single phase Pilot plant Ice-cream Upflow filter

Single phase Laboratory scale Cheese whey Upflow filter

Single phase Laboratory scale Raw milk quality

control laboratory

Upflow filter

Single phase Laboratory scale Cheese whey UASB

Single phase Laboratory scale Cheese UASB

Single phase Laboratory scale UASB

Single phase Laboratory scale Cheese whey Hybrid bed

Single phase Anaerobic SBR

Two phase Fluidized bed

Two phase Laboratory scale Milk and cream

bottling

CSTR + upflow fil

Table 6

Typical operating conditions for anaerobic digesters [2]

Anaerobic digester configuration Load (kg COD/(m3 day))

CSTR 0.5–2.5

Anaerobic filter 2–10

UASB 2–15

Fluidized bed 2–50

35 8C, for two-phase anaerobic treatment of cheese whey

[124]. In the upflow methanogenic filter, 95% COD removal

was attained at an HRT of 4 days, with a biogas production of

0.55 m3/kg COD removed. Changes to bacterial community

in a laboratory-scale two-phase anaerobic digestion system

treating dairy wastewater was reported in a recent study [125].

The ratio of total autofluorescent methanogens to total

bacteria varied between 0.01 and 3% in the acid phase reactor

(a CSTR), and 2–13% in the methane phase reactor (an upflow

anaerobic filter). The system provided a methane yield

between 0.30 and 0.34 m3 CH4/kg COD removed during the

entire operation.

In both conventional—single and two-phase anaerobic

treatment processes, primary objectives are consistent:

achievement of a high degree of waste stabilization and a

high conversion of waste to methane. Furthermore, for a

particular type of wastewater, in order to achieve these goals

specified, the maximum loadings applicable and possible

treatment rates for a particular reactor configuration are

obviously also important. For anaerobic treatment of dairy

wastewaters, the maximum loadings and corresponding

treatment rates achieved with various reactor configurations

in literature are briefly summarized in Table 5. Typical

operating conditions for anaerobic digesters are also

outlined in Table 6 [2]. High influent substrate concentra-

tions, fluctuations in dairy wastewater flow rate and

ter

Maximum loading Maximum removal Reference

6 kg COD/(m3 day) >85% COD [4]

2.82 kg COD/(m3 day) 97% COD [14]

7.5 g COD/(l day) 85–90% COD [17]

16.1 kg COD/(m3 day) >99% COD [54]

21 kg COD/(m3 day) [65]

18 kg COD/(m3 day) [66]

>20 kg COD/(m3 day) [71]

5–6 kg COD/(m3 day) >90% COD [73]

38.1 g COD/l

(as influent concentration)

[77]

>45 g COD/(l day) 70–80% COD [78]

10.8 kg COD/(m3 day) [85]

11 kg COD/(m3 day) >95% COD [87]

6 g COD/(l day) [91]

9.4 kg COD/(m3 day) 76–92% COD [119]

ter 23–7 kg COD/(m3 day) 90% COD, 95% BOD5 [123]

Retention COD removal (%)

1–5 days 80–90

10–50 h 70–80

8–50 h 70–90

0.5–24 h 70–80


composition, excessive suspended solids and lipid concen-

trations in dairy effluents, the presence of sufficient amount

of alkalinity in anaerobic digesters and appropriate

anaerobic reactor configuration employed for treatment

are common factors that affect maximum loading rates and

expected treatment efficiencies [4,14,66,73,77,78,81,116,

123]. High variability in composition and flow rate of dairy

wastewaters are probably the most important parameters that

should be considered initially. Thus, installing an equaliza-

tion basin prior to anaerobic/aerobic treatment facility is

usually required for stable process efficiency. High

suspended solids concentrations in dairy effluents particu-

larly affect treatment performances of anaerobic CSTRs and

filters [2,4]. High substrate concentrations were reported to

decrease UASB reactor performance treating dairy waste-

water [77,81]. Biomass granulation must definitely be

achieved for stable and satisfactory UASB reactor operation

[76,78]. Moreover, dairy wastewaters usually contain

differing proportions of fats and proteins that require longer

time for hydrolysis [101]. Therefore, larger reactor volumes

may be required to treat larger volumes of wastewater at

high HRTs for such dairy effluents. Presence of lipids in

single-phase anaerobic filter treatment for dairy effluents is

also a common problem, because anaerobic filters remove

lipids by entrapment, without biodegradation. This may

soon result in channelling and clogging, with a subsequent

decrease in reactor performance. Choice of appropriate

packing materials in upflow anaerobic filters also affect

maximum substrate loading rates and expected treatment

rates significantly [63,65]. Poor biodegradation of milk-fat

was attributed to the limiting rate of liquefaction, indicated

by a low hydrolysis constant in an expanded granular sludge

bed (EGSB) reactor [36]. Utilization of two-phase anaerobic

treatment processes seems a favourable choice to overcome

suspended solids and particularly lipid problems for

concentrated dairy wastewaters [122]. Besides, a two-phase

system will protect the methanogens in the methane reactor

from inhibitory levels of pH and high amounts of VFAs

produced in the first-acid reactor. Acidogenic phase will

Table 7

Anaerobic/aerobic treatment performance levels for dairy wastewaters

Effluent type System configuration

Milk bottling plant DAF + upflow anaerobic filter (UAF)

Cheese whey Downflow–upflow hybrid

anaerobic reactor (DUHR) + SBR

Cheese wastewater UASB pond + aerated pond

Synthetic milk powder/butter

factory wastewater

AAO activated sludge

Wastewater from an industrial

milk analysis laboratory

Anaerobic filter + SBR

allow pH control in the first phase, if required, in a pilot or

full-scale treatment plant [86]. Finally, sufficient amount of

alkalinity is frequently discussed to be the most important

factor for controlling process reliability during anaerobic

treatment of dairy wastewater [66,73,78,87].

5. Anaerobic/aerobic treatment of dairy wastewater

Aerobic treatment processes are commonly used together

with anaerobic processes for dairy wastewater treatment, in

order to achieve the effluent discharge limits for agro-industry

wastewaters. Results for anaerobic/aerobic treatment of dairy

wastewaters are outlined in Table 7. Among the studies

mentioned in this paper, pre-treatment of a dairy wastewater

from a milk bottling plant was reported using a pilot-scale

dissolved air floatation (DAF) unit and a pilot-scale anaerobic

upflow filter reactor [4]. The aim was to attain a BOD5

reduction of between 38 and 50% and a SS reduction of 60–

75% in the DAF unit prior to the biological treatment step. The

upflow anaerobic filter reactor achieved BOD5 and COD

reduction rates greater than 90 and 85%, respectively, with a

biogas yield of 0.40 l biogas/g COD removed. A laboratory-

scale anaerobic–aerobic biological process was used to treat

cheese whey [12]. The anaerobic downflow–upflow hybrid

reactor (DUHR) achieved 98% COD reduction at an OLR of

10 g COD/(l day). Post treatment was subsequently per-

formed using a SBR, resulting in more than 90% of both COD

and nutrient removal rates. Furthermore, full-scale anaerobic/

aerobic treatment of cheese wastewater by a system

comprising a grease trap, an UASB type pond, an aerated

pond and an effluent polishing pond was reported [13].

Reduction rates in BOD5, COD, TSS, and oil and grease were

98, 96, 98 and 99.8%, respectively. In addition to these

findings, a low cost treatment system was proposed to reduce

the strength of a dairy waste [126]. This three-stage treatment

system consisted of a sump in which the anaerobic digestion

takes place, an aerobic vegetated filter, and an irrigated

plantation. Biological phosphorus removal from a cheese

Removal Application status Reference

38–50% BOD5 (DAF)

>90% BOD5 (UAF) Pilot scale [4]

>85% COD (UAF)

98% COD (DUHR)

>90% COD (SBR) Laboratory scale [12]

98% (BOD5)

96% (COD) Full scale [13]

98% (TSS)

>90% (COD) Laboratory scale [127]

98% (COD), 99% (nitrogen) [131]


factory effluent was reported using a bench-scale SBR [127].

The existing industrial-scale treatment facility at the factory

consisted of an anaerobic equalization tank, followed by an

UASB and aerated lagoons. The effluent orthophosphates

concentration was around 5 mg/l in the bench-scale SBR

effluent, pointing out a better treatment result than that of the

industrial-scale treatment facility. A laboratory-scale acti-

vated sludge system was used to treat synthetic wastewater

from a milk powder/butter factory [128]. The substrate is fed

to anaerobic and anoxic sectors, in series with an aerobic

reactor, while the sludge is returned to the anaerobic selector,

and the mixed liquor from the aerobic sector is recycled to the

anoxic sector. Over 90% COD removal efficiency could be

achieved at an HRTof 7 days and at a nominal sludge age of 20

days in the system. Treatment of a high-strength dairy

wastewater was reported using a low-rate anaerobic pre-

treatment process and an aerobic polishing step, in an HRT

range of 5–7 days in a pilot-scale study [129]. Microbial

removal from dairy wastewater was also reported recently,

using a combined treatment system consisting of paired solids

separators, anaerobic lagoons, aerobic ponds and constructed

wetlands cells [130]. The authors reported that high turbidity

levels in dairy waste stream decreased the capability of the

treatment system, in terms of removal of some microbial

indicators and pathogens. Wastewaters from an industrial

milk analysis laboratory were treated using an anaerobic filter

(AF) and a SBR operated in series [131]. Effluent soluble

COD and total nitrogen concentrations were below 200 and

10 mg/l, respectively. Moreover, the authors concluded that

the combination of an AF and a SBR resulted in a lower

energy consumption and sludge generation. A wheat straw

biofilter was operated in a sequential aerobic–anaerobic mode

in a temperature range between 8 and 14 8C, to treat

wastewater from a milkhouse and milking parlour [132]. The

attenuation of pollutants in dairy wastewater for TSS, oil and

grease, and COD were determined to be 89, 76 and 37%,

respectively. Biological phosphorus removal from a synthetic

phosphorus-rich dairy wastewater was evaluated using an

anaerobic reactor and an activated sludge reactor [133]. The

system resulted in a final sludge phosphorus content of 4.9%

mg P/mg TSS. Utilization of aerobic/anaerobic membrane

bioreactors coupled with anaerobic digestion seemed to be a

feasible method for treating wastewaters from livestock

operations, such as dairy wastewater [134]. It was reported

that high-quality reusable water can be produced using

membrane bioreactors in the treatment of such wastewaters.

Recently, biological treatment of dairy wastewater was

evaluated in a laboratory-scale work, using anaerobic and

aerobic sequencing batch reactors [135]. The SBR system was

found to result in effective denitrification by the authors.

6. Conclusions

Conventional anaerobic treatment processes are often used

for treating dairy wastewaters. Particularly anaerobic filters

and UASB reactors are the most common reactor configura-

tions employed. In fact, the UASB reactors are very suitable

for treating food industry wastewaters, since they can treat

large volumes of wastewaters in a relatively short period of

time. More research should be directed towards treatment of

dairy wastewaters in pilot and full-scale UASB reactors in

near future, to make use of these potential advantages

outlined. Lipid degradation and inhibition in single-phase

anaerobic systems is frequently discussed in literature, since

lipids are potential inhibitors in anaerobic systems, which can

often be encountered by environmental engineers and

wastewater treatment plant operators. Moreover, high

concentrations of suspended solids in dairy waste streams

can also affect the performance of conventional anaerobic

treatment processes adversely, particularly the most com-

monly used upflow anaerobic filters. Thus, two-phase

anaerobic digestion processes should be considered more

often to overcome these problems that may be experienced in

conventional single-phase design applications, since two-

phase anaerobic treatment systems are reported to produce

better results with various industrial wastewaters, such as

olive oil mill and food-processing effluents, which are high in

suspended solids and lipids content. When two-phase

anaerobic digestion processes are evaluated as a whole, it

is clear that the acid phase digestion of dairy wastewaters is

actually investigated in various aspects. However, data

especially for full-scale two-phase applications for dairy

effluents in literature is scarce. Furthermore, in addition to

degradation of lipids, protein solubilization should be

investigated more comprehensively during acid phase

digestion of dairy wastes with a relatively high protein

content, because there is contradictory information in

literature about protein solubilization with different waste-

water types during anaerobic acidogenesis. Since high rate

anaerobic treatment of dairy wastes (or any industrial

wastewater) with a relatively higher content of particulates,

fats and proteins can often be problematic, modelling studies

simulating biodegradation mechanisms of these components

should extensively be explored. Removal of nitrogen and

phosphorus from dairy wastewaters has recently gained

significant attention, due to more strict environmental

regulations, so current research efforts clearly seem to focus

on this particular topic. Recently, bench–pilot and full-scale

applications of combined treatment methods for nutrient

removal from dairy waste effluents are frequently encoun-

tered. It is obvious that as the regulations for discharge of

nutrients become more strict in time, new modifications in

existing treatment plants will eventually be incorporated.

Finally, since the anaerobic digestion process is an imperative

tool for the production of clean energy sources, such as

hydrogen and methane, biogas production from high-strength

dairy industry wastes will always be of paramount

importance, as a valuable renewable energy source, for both

developed and developing countries in future. Particularly,

production of hydrogen by acidogenesis of high-strength

dairy waste effluents is currently worth investigating.


Acknowledgements

The authors wish to express their gratitude to the financial

support by the Bogazici University Research Fund through

project number 01Y101D.

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