SUGGESTED OMWW TREATMENT PROCESSES
As can be understood from its synthesis, OMWW possesses a double nature.
It is a strong pollutant and at the same time a possible source of valuable components, such as polyphenols, flavonoids, anthocyanins, inorganic trace elements, etc., that could be isolated (removed) and economically exploited.
Research is oriented toward flexible and efficient treatment solutions that could
ensure the detoxification of the waste compensating high capital and operating costs with the possibility of recovering and recycling some valuable components.
According to these, the numerous treatment processes that have been proposed so far could be classified in the following categories:
. Detoxification processes.
. Processes that aim at the production of various products.
. Integrated processes aiming at energy recovery.
. Combined processes.
. Detoxification Processes
These are processes that aim at ‘‘cleaning’’ the waste so as to allow its safe, subsequent disposal at water or soil reservoirs.
The most important are biological and physicochemical processes.
. Biological Processes
Biological processes use microorganisms to break down the chemicals present in
They are divided into aerobic and anaerobic processes according to the
type of the microflora used.
Aerobic processes can operate efficiently only if the concentration of the feed
is relatively low; i.e., of the order of 1 g COD=liter.
Higher concentrations can be tolerated only if the plant operates at a long hydraulic retention time or=and with high recycle ratio; both possibilities are uneconomical for a treatment plant.
Also, the aerobic treatment of concentrated wastewaters yields huge volumes of excess secondary sludge that has to be removed from the system.
At last, it is very difficult using aerobic processes to reach the required removal efficiency of pollutants such as polyphenols and lipids. For all the above reasons, aerobic processes are unsuitable for direct and efficient treatment of OMWW.
They can be used as pretreatment or posttreatment steps to increase the efficiency of the main treatment process used.
Anaerobic digestion consists of a series of microbiological processes that
convert organic compounds into methane and carbon dioxide.
Although a pretreatment or posttreatment step is also needed, anaerobic treatment is considered most suitable for OMWW detoxification.
The most important reasons for this choice are the feasibility to treat wastewaters with high organic load, such as OMWW, the low energy requirements, the production of methane that may be exploited after suitable treatment, the production of significantly less waste sludge (than aerobic processes), and the ability to restart easily after several months of shut down (Niaounakis and Halvadakis, 2004; Rozzi and Malpei, 1996).
.a. Anaerobic Processes.
Anaerobic processes are driven mostly by bacteria and have three major steps:
In the first stage, anaerobic bacteria hydrolyze complex organic compounds, such as polysaccharides and polyphenols to their monomers (simple sugars and phenols, respectively).
These molecules are converted into organic acids such as acetic, lactic, and formic acids and alcohol by acetogenic bacteria during the second stage of the process.
In the last stage, methanogenic bacteria, which are characterized by their sensitivity to pH and temperature changes, convert the organic acids into biogas (a mixture of 60–80% methane and other gases, mainly carbon dioxide) (Sabbah et al., 2004).
Anaerobic processes are affected by temperature, retention time, pH, H2 partial
pressure, the chemical composition of the wastewater, and the quantity of toxic
The process usually takes place under thermophilic or mesophilic conditions.
Retention time varies between 10 and 35 days and pH must be controlled, because acetogenic bacteria tend to lower it and methanogenic
bacteria are sensitive to pH variations.
Several technologies have been tested, including upstream anaerobic sludge
blanket reactor (UASB), contact reactors, anaerobic filters (upstream and downstream), anaerobic baffled reactors (ABR), and two-stage systems that separate acidogenesis and methanogenesis processes (Azbar et al., 2004; Borja and Gonzalez, 1994; Dalis et al., 1996; Rozzi and Malpei, 1996; Zouari, 1998; Zouari and Ellouz, 1996).
UASB-type reactors and anaerobic filters are suitable for high volumetric
pollution loads (5---15 kg COD=m3 day). COD removals of 80% and 60–65%,
respectively, have been reported but in both cases a high dilution ratio is required(1=8 and 1=5) that raises operational costs.
Anaerobic filters require very little process control and 75% reduction of phenols has been reported (Dalis et al., 1996).
Compared to contact reactors, greater production of methane and elimination of
mechanical mixing, settling, and return of the sludge has been reported (Borja and Gonzalez, 1994). Contact reactors can operate at higher feed concentrations (up to60 g COD=liter) with COD removal efficiencies greater than 80% but only at low loading rates (<5 kgCOD=m3 day) (Rozzi and Malpei, 1996).
A general problem encountered with anaerobic digestion of OMWW is that
both the addition of alkali substances to neutralize pH and of substances that are
sources of nitrogen such as urea or ammonia are necessary.
The anaerobic microflora also shows limited efficiency in the removal of aromatics, particularly condensed tannins.
Finally, scaling up these processes proves to be extremely difficult.
Growth rates of anaerobic microorganisms are appreciably lower than those of
aerobic ones and their metabolic degradation pathways require several different
microbial populations in series which make process control and stability very
delicate (Mechichi and Sayadi, 2004).
All these constraints make the use of pretreatment or posttreatment of anaerobic digestion necessary.
Pretreatment methods proposed so far include dilution of the waste, gravity settling, sand filtration, centrifugation, adsorption, membrane processes, physicochemical treatments (Ca(OH)2, NaOH, Na2CO3, Fenton’s reagent, etc.), and aerobic degradation (Sabbah et al., 2004; Zouari, 1998).
As mentioned before, strong dilution of the waste is necessary in most of the types of anaerobic digesters.
Some researchers have proposed that if OMWW is mixed with another organic effluent, definitely a more economic dilution media than water, it will also become enriched in its limiting nutrients such as nitrogen and neutralized without the addition of chemicals.
At locations where the polluting load due to olive industry is comparable or lower than domestic sewage load, OMWW can be treated in conventional domestic sewage digesters if mixed with this effluent.
Rozzi and Malpei (1996) and Marques (2001) studied the combined treatment of OMWW with piggery effluent, where no chemical correction was needed and 70–80% COD removal was achieved, but decolorization of the waste was not sufficient.
The produced effluent could be used as irrigation water.
b. Aerobic Processes.
The combination of aerobic and anaerobic treatment is extensively studied, as there are aerobic consortia that grow on undiluted or diluted OMWW and are capable of metabolizing and removing its aromatic compounds.
Fungi of this kind have ligninolytic enzymes and can degrade phenolic substances of OMWW that have structural relationships with lignin. However,
the majority of phenolic compounds removed are simple monomers, whereas
polymerized molecules such as tannins degrade more difficultly.
This happens because these compounds adsorb strongly to mycelia and extracellular enzymes so that their biodegradation is not possible.
In all cases, a certain pretreatment is necessary (dilution, thermal treatment,
etc.) and the resulting effluent always needs additional treatment before it could be safely disposed off. So, aerobic processes alone are not effective enough for the
detoxification of OMWW.