Prospects of using peat for treatment of highly concentrated industrial wastewater
The technology of deep purification of highly concentrated industrial sewage by means of an accessible and cheap natural sorbent, peat, is offered. The phenomena of application of peat in practice of wastewater treatment are considered and the technical solutions for their elimination are proposed.
Intensive development of small and medium-sized industrial enterprises, the use of modern advanced technologies in production processes has led to a decrease in specific water consumption and, consequently, the formation of highly concentrated wastewater, which quite often require local treatment even before discharge into the sewerage system of the settlement. Only physical-mechanical local treatment usually does not provide the expected results and does not ensure the preparation of wastewater for full biological treatment, especially when it comes to highly concentrated wastewater from meat and dairy industries, which contain complex polydisperse systems with colloids and dissolved organic matter. In these cases, predominantly complex technological schemes, expensive reagents and flocculants, energy-intensive and cumbersome facilities are used, significantly affecting the cost of the final product. Elimination or reduction of the specific weight of these drawbacks is a very urgent problem.
One of the most promising and effective methods of treatment of highly concentrated wastewater of meat and dairy industries is their treatment by electric current. Electrochemical methods of electroflotation and electrocoagulation are particularly suitable. The advantages of these methods lie in the property of electric current to electrolytically decompose water and dissolved impurities. Electrochemical processes create conditions for simultaneous dosed flotation and coagulation processes. In the process, the waste water is cleared of suspended, dissolved and emulsified substances.
The anodic dissolution of metals in aqueous solutions under the influence of an electric current leads to the formation of their hydroxides, which precipitate as insoluble flakes, capturing insoluble impurities. The advantages of electrocoagulation over conventional reagent methods are the compactness of the unit and the possibility of full automation of the technological process. The main disadvantage of this technology is increased energy costs, so its practical use is promising only for small facilities.
On the recommendations of the Department of drainage, combined heat and power supply and ventilation of the National University of Water Economy and research and production company "AQUA-U" (Rivne), research on the application of electrocoagulation-flotation methods for local wastewater treatment from casein production department of Ratnivka milk plant has been conducted. Application of electrocoagulation-flotation only showed that effectiveness of cleaning from contaminants and organic substances was only 50 - 60%. This can be explained by the peculiarities of pollution, as the aqueous system of casein wastewater is a rather complicated suspension, containing not only suspended substances, but also a wide range of emulsified, colloidal and dissolved inclusions of organic origin. Application of the filter after the electrocoagulator-flotator will increase efficiency of casein wastewater treatment up to 75 - 90% for suspended and organic substances. Moreover, the use of the filter for "raw" wastewater was not acceptable due to the short duration of the filter cycle and a significant increase in losses of head. The filtering surface of the loading granules was quickly covered with a film of organic substances, which were badly washed off during washing, decayed and were a source of unpleasant smell.
The application of the above technology (electrocoagulation-flotation-filtration) requires significant costs for local treatment and suggests that with additional treatment it is possible to obtain water with such an amount of impurities that it can be discharged directly into water bodies or used in technical water supply systems and thus not to spend money on transportation and treatment of waste water in municipal wastewater treatment plants. At the same time the additional treatment should be relatively cheap. In our opinion, such increase of treatment efficiency at the stage of filtration can be achieved by using (along with traditional inert filtering loads) such loading, which would be non-deficit, cheap and should ion-exchange and sorption properties.
Milled peat meets these requirements to a sufficient extent. In its exchange-sorption properties peat is similar to ordinary zeolites, glauconite, cellulose, charcoal and some resins [3-5].
Peat is a polydisperse system consisting of plant residues, and humus - plant residues that have lost their cellular structure, and products of decomposition, mainly humic substances.
The interwoven structures of plant remains, spatial and colloidal structures of peat absorb large quantities of water and result in the adsorption properties of peat.
Extreme heterogeneity of composition and structure determines the complexity of the mechanism of moisture absorption. Adsorption at different stages occurs by different mechanisms and at different rates (Fig. 1 a).
The water absorption of peat depends on its degree of decomposition, composition (ratio of plant remains to humus) and temperature. With increasing decomposition the water absorption decreases proportionally. So, water absorption of sorbent from sphagnum peat with increasing degree of decomposition from 5 to 20% decreases almost twice.
Ion-exchange properties of peat, due to the presence of highly dispersed fraction - colloidal substances, mainly represented by humic substances (HS), whose molecular weight varies from 80 to 300 thousand A.u. and the size of the particles - in the range of 6 - 14 nm.
GRs (humic acids, fulvic acids and their salts) are organic high-molecular surfactants, the macromolecules of which are built from the polynuclear aromatic and protein fragments, different in structure, molecular weight, number and type of functional groups.
The ion exchange properties of GRs are due to the large number of carboxylated ─СООН (1.3 - 3.1 meq/hr) and phenolic ─ОН (3.2 - 6.2 meq/hr) groups. Carboxyl groups account for more than 50% of the ion exchange, as phenolic hydroxyls are practically undissociated at pH <6 and only enter into ion exchange at pH = 9 - 10.
Physico-chemical properties of peat, on which its application as an adsorbent and ion exchanger is based, are determined by the state of colloidal substances.
The macromolecules of humic substances, due to hydrogen bonds and polyvalent ions, can "cross-link" with the formation of spatial structures, which are able to absorb and retain a large amount of water with substances dissolved in it, explains their adsorptive properties. Located both on the outside and inside of the spatial structures, functional groups of humic substances are capable of dissociation to form humate polyanions ; Нu(COOH)х Hu(COO−)х + хH+; Нu(COOK)х → Нu(COO−)х + хK+ ;which form colloidal particles of peat (Fig. 2). The ionized functional groups give the nucleus of the colloidal particle a negative charge and form a potential-initiating layer. Part of the counterions (NH4 +, H +, K +, Ca2 +, Mg2
+, Fe2 +, Al3 +, etc.), which lie directly on the potential of the initial layer, form a counterion adsorption layer. The nucleus with the adsorption layer forms a negatively charged colloidal particle. Having an electric charge of the same sign, the colloidal particles are mutually repulsed as they approach each other, which leads to aggregative stability of the colloidal system.
Figure 2. Structure of the colloidal particle of humic substances
Fig. 3. Dependence of φ-potential on distance and ζ-potential on ionic strength of solution
The negative charge of the colloidal particle is lower than its nucleus, therefore the remaining counterions are attracted weaker, are at a greater distance from the nucleus, and form a diffuse layer. They are much more mobile than the cations of the adsorption layer. The colloid particle forms an electrically neutral micelle with the cations of the diffusion layer.
On the interface between the colloidal particle nucleus and the layer against the ions a double electric layer is formed,
and, as a consequence, a potential difference - the surface potential φ (Fig. 2.). The relative movement of the solid and liquid phases causes the micelle to collapse along the diffuse layers. The dispersed phase and the dispersed medium acquire opposite charges, and an electrokinetic potential, or ζ (zeta) potential, arises at the interface of the rupture. The electrokinetic potential ζ for peat has a value of -2 ÷ -10 mV. The low value of ζ for peat is due to the bulk arrangement of ionogenic groups in spatial structures whose negative charge is reduced by metal ions and hydrogen ions. The higher
value of the ζ-potential, the colloidal solution is resistant to coagulation. The cations of the adsorption and diffusion layers (metal cations, H + and NH4
+) can be exchanged with solutions in equivalent quantities for any other cations, on which the use of peat as an ion exchanger is based.
Increasing the concentration of dissolved substances leads to a decrease in the - potential and stability of the colloidal solution. At a high concentration of polyvalent metal ions in the dispersion medium, the diffuse layer shrinks, the charge of the colloidal particles is neutralised, they do not repel each other, they stick together and the macromolecules are enlarged (coagulated) with the formation of a rough dispersed phase with its subsequent deposition. When peat is used for wastewater treatment, coagulated GR in the process of sedimentation captures contaminant particles and translates them into sediment.
Depending on the acidity, the colloidal system may be in different states. In an acidic environment, spatial structures predominate, which absorb significant amounts of water with dissolved substances. In an alkaline environment, the spatial and coagulation structures of the GR are broken down to individual macromolecules, which leads to increased leaching. Thus, soluble salts of humic substances (salts of monovalent cations) can be a source of secondary pollution of treated water. Thus, the ability of peat to absorb ammonia is a negative factor when using it as a sorbent for wastewater treatment because more than 70% of the absorbed ammonia reacts with humic substances with the formation of water-soluble ammonium humates (level 1,2), which are washed out of the sorbent, degrading its quality and repeatedly polluting water.
Consequently, by changing the pH of the medium and the concentration of polyvalent metal ions, adsorption and ion-exchange processes can be controlled in a targeted way. Active ion-exchange sorption occurs at pH 6.5 - 12 due to the release of dissociated carboxylic groups, spent for the construction of spatial structures, and activation of phenolic groups.
A feature of peat as an ion exchanger is the high mobility of its structures due to the swelling of the organic components of peat in the process of ion exchange, as well as the different ability to exchange carboxyl groups located on the surface and inside the spatial structures.
The high effectiveness of the use of peat for the purification of wastewater from metal ions can be evidenced by the values of the absorption capacity (the sum of all cations absorbed by peat) for different types of peat (Table).
Table
Absorption capacity of peat and its components (mmol / 100 g) [7]
[7]: Na+ < NH+4 < K+ < Mg2+ < Ca2+ < Ba2+ < Al3+ < Fe3+ < H+.
From the above data we see that the absorption capacity is the higher the
the greater the atomic weight and valence of the metal. H+ cations are better absorbed and are more difficult to be displaced from the peat by other cations. The use of hydrochloric acid to regenerate peat-based adsorbents is based on these properties.
In spite of the high efficiency of peat-based adsorbents in the process of wastewater treatment from oil products, synthetic surfactants and ions of polyvalent metals, so far peat as a sorption material has not been widely used because it has a number of disadvantages, which appeared during its application, namely: high swelling in water, poor permeability of the liquid flow, which significantly worsens the hydraulic characteristics of the filter load; washout of peat fibres in the filtration process; peat components washing out.
To eliminate or reduce the impact of these shortcomings in wastewater treatment can be the following ways: the use of cut peat to adsorb impurities only in water clarification facilities (flotators, settling tanks, clarifiers) using in filters thin layers of cut or pelletized peat or technical means that prevent compaction and sticking of peat. But if peat is used to adsorb contaminants only in water clarification facilities, a significant amount of it will be used inefficiently for those contaminants that are removed without the use of peat. It is interesting to perform filtration through a thin layer of peat. The thickness of this layer is determined primarily by the hydrodynamic characteristics of the filter and the duration of the filter cycle. According to studies carried out on actual runoff from a casein production plant, it is recommended that the thickness of milled peat during wastewater filtration should be no more than 5 - 10 cm. In this case, the head losses and the duration of the filter cycle were acceptable under these real technological conditions.
A promising approach is to improve the properties of peat as a filtering sorption material by means of its granulation by the rolling method [1]. Real wastewater, which according to the design was to be supplied to the cationic-exchangers, was used in the research. Peat with a degree of decomposition of 35 - 40% and a moisture content of 21.5% was used for granulation. Filtration rate was assumed to be 5 - 6 m/h. Researches have shown [3], that in the process of damping, physical and technical properties of peat are improved, its bulk density increases, swelling decreases. Granulated peat has a high layer porosity which improves its hydraulic and filtration characteristics. It is found that leaching of components from the granulated peat, in contrast to the original, in the range of pH 4 - 6 does not occur, that is, low-molecular organic secondary contaminants do not enter the purified water. However, in this case it is necessary to control the humidity of the obtained granules within 25 - 30%, as when decreasing the humidity the sorption capacity of the peat will be reduced due to the irreversible compression of the spatial structures of the peat. So, the drying of granules to an absolutely dry state leads to the loss of ion-exchange capacity
capacity of peat absorption by metal ions in 1.5 - 2 times.
In the course of carried out researches for quantitative estimation of non-ferrous metals sorption on granulated peat the dynamic exchange capacity has been determined which is in meqv/l for ions: copper - 3.17, nickel
- Copper ions - 2.2, zinc - 1.92 (at pH 5 - 6), chrome - 0.1, iron - 1.2 (at pH 1.5 - 2.5) [1]. The working exchange capacity of the granulated peat reached 220 mole/m3, the degree of removal of non-ferrous metal ions was 92 - 99%, hardness cations - 50 - 80%. It is found that the granulated peat can be repeatedly used in a dynamic mode with regeneration by hydrochloric acid. At deterioration of performance peat is unloaded from the filter, after which it can be used as fuel.
It should be noted that pelletised peat can be competitive in wastewater treatment technologies, where ion exchange filters are provided. For small objects, especially at local treatment, such technology is difficult and therefore, in our opinion, the use of ordinary granulated peat as a sorption load with the use of technical means that counteract its compaction and sticking in the filters is promising. Thus, abroad for wastewater treatment at small sites such as cottages, small multi-storey buildings, restaurants, laundries, etc. filters filled with sphagnum peat are used. Such filters, developed by the Canadian company Premier Tech. are produced on an industrial scale by Ecoflo @ Biofilter [8].
We propose to feed wastewater into the filtration layer from ordinary cuttings peat (Fig. 4). The resulting fluidised bed will provide reasonably good contact conditions for the wastewater and the peat particles. As a consequence, one should expect better sorption and ion exchange properties of this type of loading with minimum head losses.
It is advisable to arrange two-layer loading, where the upper layer is a peat load and the lower one is granular. In this case it is possible to use mesh drum filters, which simplifies the filtration technology. Washing water from the filter with spent peat loading should be discharged into the wastewater clarification facilities.
The proposed technology of wastewater treatment with the use of peat due to the availability and low cost of the initial natural raw material while providing highly effective treatment can find a wide practical application, especially for small facilities with highly concentrated wastewater.