Proceedings of the 13th International Conference on Environmental Science and Technology
Athens, Greece, 5-7 September 2013
EFFECTS OF IRRIGATION OF DAIRY FACTORY EFFLUENT ONTO LAND ON
SOIL PROPERTIES
R.J. HAYNES and Y-Y. LIU
School of Agriculture and Food Sciences/CRC CARE, The University of Queensland, St
Lucia, Queensland 4072, Australia.
EXTENDED ABSTRACT
The dairy industry is a major source of food processing wastewater. Dairy factory effluent
(DFE) contains a high organic load due to the presence of diluted milk/milk products and
also contains significant quantities of cleaning products such as NaOH, NaOCl and
H3PO4. The effluent therefore has a high content of Na and phosphate as well as soluble
organic matter. It is often irrigated onto land used for dairy production that surrounds
factories. Analysis of soils from field sites receiving long-term applications of DFE showed
a large accumulation of exchangeable Na and extractable P, a loss in exchangeable Mg
and a decrease in soil aggregate stability (caused by the dispersive effects of Na). DFEirrigated soils also had increased microbial biomass C and N and basal respiration but
there was no change in organic C content compared to unirrigated controls. The
widespread accumulation of Na and P in soils under DFE irrigation has led some
environmental agencies to suggest that NaOH as a cleaning agent should be replaced by
KOH and H3PO4 by HNO3 or an organic acid such as CH3COOH. A short-term incubation
experiment compared the effects of addition of synthetic DFE containing H3PO4, HNO3
and CH3COOH and it showed that, irrespective of acid source, DFE irrigation caused an
accumulation of exchangeable soil Na, a loss of exchangeable soil Ca and significant
increases in microbial biomass C and basal respiration. Where H3PO4 was the acid
source, P accumulated in the surface soil. Due to the introduction of an additional C
source, the use of CH3COOH resulted in a higher soil microbial biomass than the other
two acids. A 16-week greenhouse study was carried out to compare the effects of
irrigation with synthetic DFE containing H3PO4/NaOH, H3PO4/HNO3/NaOH or
CH3COOH/KOH on soil properties. The cumulative effect of DFE addition was to increase
exchangeable Na, K, Ca, Mg, exchangeable Na percentage, microbial biomass and basal
respiration in the soil. Replacement of NaOH with KOH resulted in a greatly increased
accumulation of exchangeable K in the soil. The effect of added NaOH and KOH on
promoting breakdown of soil aggregates during wet sieving was, however, similar.
Growth of ryegrass was increased by additions of DFE other than that containing
CH3COOH. It was concluded that replacement of H2PO4 by HNO3 is a viable alternative
but that CH3COOH can have phytotoxic effects. Replacement of NaOH with KOH lowers
the likelihood of phytotoxic effects of Na but K and Na have similar effects on dispersion
and disaggregation. Fortunately, the high organic matter content of pasture soils means
they are highly stable so dispersion does not usually occur.
KEYWORDS: dairy factory effluent, pastoral soils, soil nutrient status, soil microbial
activity
1. INTRODUCTION
Dairy factory effluent (DFE) contains soluble organic matter (mainly milk residuals) and
inorganic components which are primarily residual cleaning agents used in the factory
(e.g. NaOH, H3PO4). In Australasia, and in many other parts of the world, dairy factories
in rural areas commonly irrigate effluent onto surrounding pastoral land (Speir, 2002; Liu
and Haynes, 2011a). This is not only a waste disposal strategy for the factory but also
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increases productivity of surrounding land since it is a supply of irrigation water (often a
scarce resource) and nutrients (e.g. N and P) to farmers without cost.
Nonetheless, a number of environmental concerns surround land application of DFE.
Apart from factors such as nuisance odours and aerosol drift, they include excessive
accumulation of Na+, possibly promoting soil dispersion, and excessive accumulation of P
promoting runoff of P to the surrounding environment (UNEP 2004; Liu and Haynes
2011a). The high Na content of dairy factory effluent not only arises from the use of
NaOH, but also NaClO as a cleaning agent, as well as from the extensive use of NaCl in
dairy processing (e.g. cheese making). The concerns regarding P accumulation have
prompted reductions in the use of H3PO4 in some factories who have replaced a portion
of it with HNO3. Suggestions have been made that use of biodegradable organic acids
(e.g. acetic acid) would further increase the eco-efficiency of the dairy industry (UNEP
2004; Liu and Haynes 2011a). It has also been proposed that replacement of NaOH by
the more expensive KOH would be another eco-efficient move (UNEP 2004) since K is a
fertilizer nutrient routinely applied to pastures. Despite such suggestions, there are no
published studies investigating how changes in the acid or alkaline content of DFE affects
the quality of DFE-irrigated pasture soils or pasture growth and nutrient uptake.
The purpose of this paper is to review recent research conducted in this laboratory into
the effects of applications of DFE on soil chemical and microbial properties. The research
includes three main experiments: (i) the effects of long-term DFE irrigation at a field site
(Liu and Haynes, 2011b), (ii) a laboratory experiment comparing the effects of different
acids as cleaning agents (Liu and Haynes, 2012) and (iii) a glasshouse experiment
comparing different combinations of acid and alkaline cleaning agents (Liu and Haynes,
2013).
2. LONG-TERM EFFECTS
Soil properties at two sites where long-term applications of DFE had been applied were
compared with those at an adjoining site that had been irrigated with river water. The
dairy factory effluent undergoes primary treatment to remove large particles and debris
prior to its use as irrigation water. The mean content of factory effluent in the 2005-2009
period was: N = 151, P = 44, K = 456, Na = 388, Ca = 63, Mg = 18, Cl = 644, BOD =
3338 and TSS = 1780 mg L-1. However, cleaning agents used in the factory were
changed in 2004. Phosphoric acid was replaced with HNO3 and NaOH with a mixture of
NaOH and KOH. Previous to that, effluent content differed greatly with contents being in
the range of: P = 90-120, Na = 600-700, N = 40-60 and K = 20-30 mg L-1. In the 20052009 period irrigation has typically been applied to fields at rates of 15-40 mm (mean =
29 mm) 4 to 8 (mean = 6) times per year. Nutrient loadings have commonly been in the
range of N = 100-400, P = 30-80 and K = 300-900 kg ha-1 yr-1. Although early records are
not definitive, during the first 10-20 years the long-term DFE irrigated fields may have
received irrigation at similar rates (i.e. about 29 mm) but about 15-20 times per year.
Since the effluent had a substantial K+ and Na+ content, accumulation of these cations in
the soil occurred (Table 1). That is, although monovalent cations are held less strongly
held on cation exchange sites than divalent ones, by mass action the added K+ and Na+
displace other cations (e.g. Ca2+ and Mg2+) into soil solution and they can then be
leached
Table 1. Effect of long-term irrigation with dairy factory effluent on exchangeable cation
concentrations in the soil.
Treatment
Exchangeable cations (cmolc kg-1)
ESP
(%)
Ca
K
Mg
Na
Control
6.6
0.22
2.2
0.13
1.36
Long-term DFE 1
6.4
1.12
1.3
1.15
11.52
Long-term DFE 2
9.5
1.39
1.7
1.22
8.84
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down the soil profile (Liu and Haynes, 2011a). A decrease in exchangeable Ca2+ and
Mg2+ is therefore commonly reported where Na+-enriched effluents have been repeatedly
applied (Menneer et al., 2001; Luo et al., 2004). At the study sites, a decrease in
exchangeable Mg2+ was evident but exchangeable Ca2+ levels were not greatly changed
(Table 1). This is attributable to the application of about 3 t ha-1 of gypsum (CaSO4.2H2O)
in early 2007 which was applied to counteract previous accumulation of exchangeable
Na+ that had occurred at the long-term irrigated sites. The elevated exchangeable Na+
noted here will therefore be the result of residual Na+ remaining after gypsum application
plus that which has accumulated since the application.
The increase in soil pH (Table 2) under DFE irrigation is attributable to the high pH of
DFE (7-8). The EC values (1:5 soil:water extracts) under long-term effluent irrigation were
0.2-0.3 dS m-1 (Table 2) which are considered moderately saline (Shaw, 1999). This salt
accumulation is the result of the salt content of DFE and a reflection of the fact that very
little rain had fallen in the 4-month period prior to sampling (mean rainfall per month over
that period was only 52 mm). Percolating rainfall will tend to leach accumulated soluble
salts out of the surface soil and down the soil profile.
In this study, exchangeable sodium percentage (ESP) reached 8.9 and 11.2% under
long-term DFE (Table 1) and the soils would be considered as sodic by Australian
standards (Sumner, 1993). The current factory practice of using a mixture of KOH/NaOH,
rather than NaOH alone, for cleaning certainly reduces future accumulation of
exchangeable Na although exchangeable Na+ and K+ have similar effects on dispersion
(Arienzo et al., 2009) since they are both monovalent cations. Values for percentage
monovalent cations (K+ plus Na+ as a percentage of total exchangeable cations) reached
19 and 25% under long-term irrigation and such values are likely to favour dispersion
(and thus degradation of soil physical properties). Indeed, when aggregates from the
various treatments were subjected to wet sieving, the percentage of stable aggregates
remaining greater than 1 mm diameter
Table 2. Effect of long-term irrigation with dairy factory effluent on some soil chemical
properties
Treatment
pH
pH
EC
Colwell P
Organic P
(water)
(CaCl2)
(mS m-1)
(mg kg-1)
(mg kg-1)
Control
5.5
4.6
4.1
61.8
580
Long-term DFE 1
7.2
6.5
31.3
624.9
526
Long-term DFE 2
7.1
6.3
20.3
457.8
511
Table 3. Effect of long-term irrigation with dairy factory effluent on soil organic matter
status and the size and activity of the soil microbial community.
Treatment
Organic C
(g kg-1)
Total N
(g kg-1)
Control
Long-term DFE 1
Long-term DFE 2
50.3
35.0
46.8
3.9
3.7
4.2
Microbial
biomass C
(mg kg-1)
465
566
708
Basal
respiration
(ug C g-1 day-1)
22.3
36.5
34.8
Metabolic
quotient
(ug C mg-1 day-1)
2.0
2.6
2.1
was: irrigated control = 82%, long-term DFE1 = 64% and long-term DFE2 = 70%.
Nonetheless, aggregate stability was still relatively high (i.e. 60-70%) in the DFE-irrigated
soils and this reflects the high structural stability of soils under permanent pasture
(Haynes and Beare, 1996).
Dairy factory effluent traditionally has a high P content, due to the use of H3PO4 as a
cleaning agent and as a result, P accumulated in the soil (Table 2). Degens et al. (2000)
previously noted a large accumulation of extractable and total P in soils under long-term
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DFE irrigation. Since there was no significant increase in soil organic matter content
under long term DFE irrigation, it is not surprising that DFE-derived P accumulated in
inorganic rather than organic form (Table 2). Such accumulation of soil P in the surface
soil could potentially result in increased losses of P via runoff and/or leaching. Indeed,
recent research has shown that when soil test P levels exceed a certain “change point”
value, soluble P increases and significant leaching losses of P can occur (Hesketh and
Brookes, 2000). Nevertheless, the change from H3PO4 to HNO3 as a cleaning agent has
meant that effluent P concentrations are now low and further P accumulation will be
minimal.
The lack of any increase in organic C content (Table 3) following long-term effluent
irrigation suggests that the additional inputs of soluble C in effluent are approximately
balanced by losses of C of a similar magnitude. In addition, the large pool of organic C in
the surface of pasture soils makes detection of small increases very difficult. Similar
results were recorded by Degens et al. (2000) and Sparling et al. (2001) on long-term
DFE-irrigated soils in New Zealand. The increase in the size (microbial biomass C and N)
and activity (basal respiration) of the soil microbial community (Table 3) is likely to be the
result of regular inputs of soluble C (e.g. lactose) in the effluent. Such an increase in
microbial biomass, despite no increase or even a decrease in organic C, was also noted
by Sparling et al. (2001) in long-term DFE irrigated pastures. There was no increase in
metabolic quotient due to DFE irrigation (Table 3) indicating that the increase in basal
respiration was proportional
Table 4. Effect of application of synthetic dairy factory effluent containing acetic, nitric or
phosphoric acids or control (distilled water) on exchangeable sodium percentage,
extractable P and the size and activity of the soil microbial community.
Treatment
ESP
(%)
Colwell P
(mg kg-1)
Microbial biomass C
(mg kg-1)
Basal respiration
(ug C g-1 day-1)
Control
Acetic acid
Nitric acid
Phosphoric acid
0.66
6.3
6.0
6.4
39
38
39
47
730
1029
959
944
72
96
87
82
Metabolic
quotient
(ug C mg-1 day-1)
4.11
3.88
3.78
3.69
Table 5. Effect of application of synthetic dairy factory effluent containing milk residues,
or milk residues plus different acid and alkaline cleaning agents on pH, EC, extractable P
and the size and activity of the soil microbial community.
Treatment
pH
EC
(mS m-1)
Colwell P
(mg kg-1)
Control
Milk residues
H3PO4/NaOH
H3PO4/HNO3/NaOH
CH3COOH/KOH
5.9
5.8
6.1
6.0
6.2
0.03
0.06
0.07
0.07
0.08
22.7
22.7
54.1
27.6
22.6
Microbial
biomass C
(mg kg-1)
1208
1683
1627
1753
1999
Basal
respiration
(ug C g-1 day-1)
21.5
25.6
29.5
27.4
29.8
to the increase in microbial biomass C. An increase in metabolic quotient is considered a
response of the microbial community to adverse conditions (either stress or disturbance)
(Wardle and Ghani, 1995). Thus, the accumulation of soluble salts, P and Na in the
effluent-treated soil did not appear to cause undue stress to the soil microbial community.
3. SHORT-TERM EFFECTS OF DIFFERENT ACIDS
The effects of addition of synthetic dairy factory effluent (DFE) containing phosphoric,
nitric or acetic acids as cleaning agents, on soil chemical and microbial properties were
studied in an 84-day open incubation laboratory experiment. At the conclusion of the
experiment, soils from DFE treatments showed a large accumulation of exchangeable
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Na+ and a large increase in ESP, a reduction in exchangeable Ca2+ (data not shown) and
significant increases in microbial biomass C and basal respiration (Table 4). The
repeated applications of soluble C in DFE resulted in maintenance of a larger, more
active (as measured by CO2 evolution) microbial community than where no DFE was
applied. Thus, the ten-fold increase in exchangeable Na, and ESP, caused by DFE
addition, did not appear to have an inhibitory effect on the size and activity of the
microbial community. Indeed, application of DFE caused a decrease in metabolic quotient
(Table 4) suggesting the microbial community was less stressed than in the control
treatment (Wardle and Ghani, 1995). Since acetate constitutes an additional readilymetabolizable C source, the size and activity of the microbial community was larger
where the HOAC-DFE was applied than where the other two DFE types were added.
4. SHORT-TERM EFFECTS OF DIFFERENT ACID AND ALKALINE CLEANING
AGENTS
A 16-week greenhouse study was carried out in which the effects of addition of synthetic
dairy factory effluent containing (a) milk residues alone or milk residues plus (b)
H3PO4/NaOH, (c) H3PO4/HNO3/NaOH or (d) CH3COOH/KOH, on soil chemical and
microbial properties were investigated.
As already noted, the increases in microbial biomass and basal respiration in DFEtreated soils (Table 5) are primarily attributable to the repeated applications of soluble C
in DFE which act as a substrate for the heterotrophic biomass. The lack of any
discernable changes in metabolic quotient (data not shown) suggest that additions of
DFE did not cause substantial stress to the microbial community. Nonetheless, over the 4
month period of the experiment a microbial community adapted to sodic soil conditions
could well have developed so that the microbial communities present in the DFE
treatments may well have differed to those present in the control soil.
Table 6. Effect of application of synthetic dairy factory effluent containing milk residues,
or milk residues plus different acid and alkaline cleaning agents on exchangeable cation
concentrations in the soil.
Treatment
Exchangeable cations (cmolc kg-1)
ESP
(%)
Ca
Mg
K
Na
Control
5.0
2.9
0.43
0.29
3.4
Milk residues
5.2
3.2
0.58
0.31
3.3
H3PO4/NaOH
6.3
4.5
0.48
1.62
12.6
H3PO4/HNO3/NaOH
6.3
4.5
0.57
1.59
11.7
CH3COOH/KOH
6.3
4.5
1.45
0.69
5.3
Replacement of NaOH with KOH resulted in accumulation of exchangeable K, rather than
Na, in the soil (Table 6). This has similar effects on dispersion of soil colloids although, as
already noted, permanent pasture soils are relatively resistant to dispersion and structural
breakdown. Indeed, a separate incubation study demonstrated that the effects of added
NaOH and KOH on promoting breakdown of soil aggregates after wet sieving (loss of
>1.0 mm dia. soil aggregates and formation of < 0.25 mm dia. particles) were similar and
both greater in an arable than pasture soil (Liu and Haynes, 2013).
Nevertheless, fertilizer K inputs can be withdrawn and accumulated K is likely to be less
detrimental to pasture plant growth than that of Na. Replacement of part of the H 3PO4 by
HNO3 minimized DFE-induced soil P accumulation (Table 5). While the rates of P applied
in H3PO4/NaOH treatment far exceeded normal fertilizer P additions to pastures, even if
all the acid was present as HNO3, the N inputs would still be in the same range as normal
fertilizer N inputs. During periods of active pasture plant growth, N inputs from DFE are
unlikely to be excessive so leakage of DFE-N to the environment will normally be
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minimal. As in the previous experiment, the use of CH3COOH as a cleaning agent
resulted in the greatest microbial biomass C values.
Dry matter yields of shoots (Fig. 1) were not increased by CH3COOH/KOH/Na. However,
applications of H3PO4/NaOH and H3PO4/HNO3/NaOH resulted in significant increases in
dry matter yield with the H3PO4/HNO3/NaOH treatment gave the highest yield. Dry matter
yields of roots showed similar trends to those for shoots. The reason that
CH3COOH/KOH-DFE gave the lowest plant dry matter yields, of all the DFE treatments,
is probably because of the phytotoxic effects of acetic acid which can occur at relatively
low concentrations (e.g. 5.0 mM) (Lynch 1977). The principal deleterious effects appear
to be suppression of root growth, but nutrient ion leakage from roots has also been
implicated (Lee 1977; Lynch 1977). As a result, caution should be exercised in
recommending the use of CH3COOH as a cleaning agent.
Figure 1. Effect of application of synthetic dairy factory effluent containing milk residues,
or milk residues plus different acid and alkaline cleaning agents, on dry matter yields of
perennial ryegrass in a 16-week greenhouse study. Means for root and cumulative shoot
yields followed by the same letter are not significantly different at P ≤ 0.05.
5 CONCLUSIONS
Addition of DFE to soils has substantial potential effects on soil and environmental quality
such as both accumulation and leaching of Na and leaching of other bases including Ca.
The effects of a change in type of acid used as a cleaning agent has only a minor effect
on these processes. Replacement of NaOH with KOH lowers the likelihood of phytotoxic
effects
of Na but K and Na have similar effects on dispersion and disaggregation. Certainly, a
change from phosphoric acid to nitric or acetic acid reduces P accumulation in the soil.
Irrespective of the type of acid used, DFE applications induce a larger, more active and
less stressed soil microbial community and use of an organic acid (e.g. acetic) magnifies
this effect. This occurs in spite of the large increases in ESP induced by DFE irrigation.
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