Author: Customer
Company: ARO

Abstract
Wastewater effluent irrigation is a common practice worldwide, especially in arid and
semi-arid countries where the fresh water supply is limited.
The main activity of most municipal wastewater treatment plants is the biological
process, also known as the secondary treatment stage. Secondary wastewater effluent can be used for irrigation according to health standard guidelines, which are determined by the method of wastewater treatment and the possibility of transfer of disease to the grower and/or consumer. "Tertiary" wastewater effluent, which is suitable for unrestricted irrigation, is derived through "deep-bed filtration" (i.e. recharge of "secondary" wastewater into the aquifer for a period of some months).
Irrigation with wastewater effluent enriches the soil with organic matter, part of which is
the dissolved organic matter (DOM), that can affect the soil characteristics and transport
micro-pollutants, such as heavy metals, through the soil profile to the ground water.
The aim of the research was to examine the effect of secondary wastewater effluent
irrigation ("effluent") versus tertiary wastewater irrigation ("tap-water") on the nature of
DOM and its copper binding capabilities.
Eucalyptus trees which were planted in 200 liter containers were irrigated with a
consistent amount of effluent or tap-water. Less water was given to the trees than they could evaporate (deficit irrigation). After 5 months of summer irrigation, the soil in the containers was rinsed and the leachates were collected. The DOM was derived from both leachates ("effluent leachate" and "tap-water leachate") after filtration with a 0.45 μm cut-off filter. In addition, DOM was also extracted from the original effluent.
The DOM was size fractioned (DOM> 1 kDa, 200 Da< DOM < 1 kDa, DOM< 200 Da),
with the use of membranes by a tangential flow technique.
Selected samples of DOM > 1 kDa were washed with sodium chloride solution using a
stirred cell as a means of exchanging the bound elements with sodium (homoionization),
and to cause maximum separation of the DOM's components.
DOM in the size fractions DOM > 1 kDa and 200 Da< DOM< 1 kDa was characterized
by 1H-NMR, FTIR, fluorescence Excitation-Emission Matrix (EEM) and fluorescence
emission spectrum.
The Cu-DOM complexation (in the DOM> 1 kDa before and after homoionization and in
the 200 Da< DOM< 1 kDa size fractions) was investigated by titration with Cu(NO3)2,
employing a copper ion selective electrode (Cu-ISE).
Changes in the fluorescence emission spectrum of the DOM after copper binding, was
used for characterization of the DOM binding sites for this metal.
It was found that in the deficit irrigation practice, the concentration of DOM in the soil
solution was 1-2 magnitude higher than that in the effluent, but there was no significant
difference in the concentration between effluent irrigation and tap-water irrigation.
Aromatic residue concentration in the DOM (< 0.45 μm) was higher in both leachates,
and lower in the effluent itself.
1H-NMR of DOM> 1 kDa showed that effluent DOM was more aliphatic and less
aromatic than both of the leachates DOM. Leachates DOM showed higher concentration of aromatic protons and lower concentration of aliphatic protons compared to their
concentrations in effluent DOM.
Effluent DOM contained more aliphatic protons in the α position to withdrawing electron
groups (i.e. double bond, carbonyl, carboxyl and aromatic ring), and more aliphatic protons on carbons bonded to the oxygen atom (i.e. ether, ester and alcohols) compared to both leachates DOM.
The FTIR analysis of DOM> 1 kDa also confirms that the leachates DOM is more
aromatic than effluent DOM, but this result is equivocal since the FTIR absorption of
aromatic substances occurs at the same frequencies as that of carboxylic acids, ketones and aldehydes which can be found in high concentration in the leachates.
No significant differences were noted in the FTIR analysis of DOM in the size fraction
200 Da< DOM< 1 kDa. The analysis showed a larger presence of alcoholic, phenolic,
esteric, etheric and polysaccharide groups, compared to the DOM> 1 kDa.
The EEM of DOM > 1 kDa and 200 Da< DOM< 1 kDa showed two significant peaks
which indicate the presence of humic substances (fulvic/humic acid-like substances). These peaks were located in the excitation/emission wavelengths of 248-278 nm /436-451 nm (peak A), and 324-334 nm/431-447 nm (peak B). The location of the peaks differed between the DOM types and the DOM size fractions.
Effluent DOM of both size fractions showed an additional peak along the
excitation/emission wavelengths of 275-280 nm/309-350 nm (peak C), which indicates a
presence of the amino acid trypthophan and represents protein-like substances, and another less noticeable peak (shoulder) apparently indicates the presence of the amino acid tyrosine and represents aromatic protein-like substances. These peaks were not identified in the leachates DOM in either size fractions.
It therefore appears that these protein substances in the effluent undergo biodegradation
during its stay in the soil.
The effluent DOM> 1 kDa bound less copper in comparison to the DOM of the two types
of leachates. Tap-water leachate DOM bound more copper than the effluent leachate DOM, apparently due to the fact that tap-water leachate DOM had a higher concentration of condensed aromatic rings and/or functional groups (i.e. carboxyl, phenol and nitro) which are bound to the aromatic structures. The functional groups that are bound to the aromatic rings are known to have a high affinity to heavy metals, including copper.
Homoionization with sodium of all DOM > 1 kDa samples reduced the copper binding in
comparison to non homoionized DOM samples in the same size fraction. It appears that
homoionization changed the spatial structure of the DOM so that less functional groups
were able to bind copper.
The copper binding capability of homoionized effluent DOM was less than that of both
leachates DOM.
The fluorescence emission spectrum of the DOM samples showed that specific emission
(per organic carbon unit) was in the following order: effluent leachate> effluent water> tapwater leachate. It appears that the low specific emission of tap-water leachate was due to a high concentration of carboxylic groups (electron withdrawing groups) which decrease the fluorescence intensity.
A decrease in the fluorescence intensity was found after the addition of copper to the
DOM solutions. Copper, which binds to DOM sites close to fluorescence centers (i.e.
carboxyl group which is bound to aromatic ring) causes a decrease in the fluorescence
intensity of these sites. The DOM-copper complex emits light and immediately goes back to the ground state without fluorescence emission.
The copper binding of effluent DOM showed a moderate decrease of the relative
fluorescence intensity (the fluorescence intensity with copper divided to the fluorescence
intensity without copper), while the copper binding of the two types of DOM leachates
showed a steep and much more pronounced decrease in the relative fluorescence intensity.
Since effluent DOM is more aliphatic than both leachates DOM, it can be assumed that
copper binding groups are not found near fluorescence centers (which are predominantly
aromatic rings), and therefore copper binding has only a small influence on the fluorescence intensity of effluent DOM.
Both leachates DOM are more aromatic and therefore it can be assumed that more of the
copper binding groups are found near fluorescence centers, and copper binding has a strong influence on the fluorescence intensity.
In summary, during the tree growing process with deficit irrigation (without rinsing),
organic matter and DOM accumulate in the soil (the original planting was in dune sand
without organic matter). The nature of the DOM which accumulated in the soil after
secondary effluent irrigation was very similar to the DOM which accumulated in the soil
after tap-water irrigation, and very different from the DOM of the effluent leachate itself.
The amount of copper that can bind to the DOM of the soil which was irrigated with
effluent water was similar (or a bit less as found in the DOM > 1 kDa) to the amount that
can bind to the DOM of the soil which was irrigated with tap-water.
It can be assumed that microbial degradation processes that occur during the presence of
the organic matter originating from the trees roots and from the irrigation water in the soil, were the main factor that had influence on the nature of the DOM.