Resultado final
The effect of the rate of acidification of the influent wastewater onto performance of anaerobic digestion of agro-industrial wastewater was evaluated. Three UASB reactors (25 litres each) were inoculated with low-grade seed sludge and subsequently fed with a synthetic wastewater consisting of sucrose, volatile fatty acids, nutrients and trace elements. The volatile fatty acids (VFA) were acetate, propionate and butyrate in the ratio 2:1:1 (based on COD).
The sucrose: VFA ratio differed in each reactor; . 1:10, 1:1 and 10:1 respectively. The reactors were started at a low loading rate (0.9-1.2 kg/m3day), that was being increased slowly to 4 gCOD/Lday. In order to maintain a required up-flow velocity and promote mixing and sludge bed expansion, re-circulation was introduced. Up-flow velocity was gradually increased. The operational temperature was 300C. After 163 days of continuous experiment the following amount of biomass was found: UASB 1 reactor - 27.1 g/L; UASB 2 33.7 g/L and UASB 3 -35.4 g/L.
The lowest CODVFA removal effect was estimated in UASB 1 reactor, where the VFA concentration were lower in comparison with the other two reactors. CODVFA removal effects in the reactors with 50 and 90% of VFA were very high: UASB 2 reactor (50% VFA) - 93.6% and UASB 3 reactor (90% VFA) - 96.3%. The sludge particles showed a great variety with prevailing fraction of small particles.
Anaerobic waste water treatment and methods to evaluate the operation and performance of anaerobic waste water treatment plants were reviewed in a detailed way. In general, protocols for waste water analysis related to anaerobic treatment do not differ from those for aerobic treatment. For anaerobic treatment however, the array of analytical methods utilized differs somewhat from those applied in aerobic waste water treatment as a consequence of the different biological conversion processes and metabolites produced.
In the first part of the manual the principles of and the background behind the use of the various analytical methods is discussed with the aim to provide with a general understanding.
In the second part, examples of analytical procedures are presented. In there, analytical methods that are described in Standard Methods, the standard reference work for the characterization of waste waters, are only briefly discussed and only a reference is given.
The analytical methods discussed in the manual are divided in 3 sections i.e.
- Analytical methods for physical-chemical characterization of waste water,
- Analytical methods for physical-chemical characterization of biomass and
- Bio-assays for the biological characterization of waste water and biomass with respect to biodegradability, activity and toxicity.
The listed methods aim to be generally applicable and represent the best current practice for analysis applied in anaerobic waste water treatment. To contribute to standardization of analytical methods and harmonization of protocols, the selected methods comply whenever possible with those described in Standard Methods, allowing uniform protocols and accurate and reliable test results.
Based on results of the agro-industry wastewaters characterisation the wastewater from the potato industry Marbo, Backi Maglic, Serbia and Montenegro, was selected as the most suitable for the natural formation of granular biomass in an anaerobic sludge bed system.
Sludge mixture from the bottom of anaerobic lagoon in which chips wastewater has been treated and sludge from the bottom of lake was selected as inoculum. Based on the measured sludge methanogenic activity (0.13-0.20 gCOD-CH4/gVSSday), lab-scale UASB reactor was inoculated with approximately 2.5 gVSS/L of wet sludge. The experimental work aimed to develop granular sludge using a laboratory- scale UASB reactor of 24 L lasted for 79 days. A design for lab-scale Up-flow Anaerobic Sludge Bed Reactor and a manual for all analyses were done at Wageningen University (P1).
A number of typical parameters was determined in the experiment : COD fractions, VFA, pH methane, NKj, alkalinity and total and volatile suspended solids.
The selected low seed sludge, mixture of the anaerobic lagoon bottom sludge and the lake bottom sludge turned out to be appropriate inoculum for the formation of granular anaerobic sludge. The results obtained, have shown that the process of sludge granulation was followed with an increase of the treatment efficiency (over 80% at volumetric loading of approximately 4 kgCOD/m3day) and biogas production. Sludge methanogenic activity increase to 0.59 gCOD-CH4/g VSSday.
The technical and economical evaluation of the anaerobic processes and technologies can not be completely evaluated only from laboratory data since scale up factors can play an effective role during full scale plant design. To evaluate the performance of an inclined plug flow reactor a pilot plant has been designed and constructed according to the experience gained from laboratory scale reactors and similar reactors installed at other research institutes. The plant has been equipped with on line record of temperature inside of reactor, effluent and gas quantity and gas quality, while measurement of total and volatile solids, pH, volatile fatty acids, alkalinity, ammonia and organic nitrogen was carried out at the HEIS laboratory.
Solid waste obtained from the potato processing industry, was past through mechanical treatment prior to treatment process. Treatment temperature was set and controlled on 36°C. Samples for laboratory analyses were taken from three sampling point along reactor chamber.
The plant was operated to evaluate the performances of the treatment in terms of maximal loading, biogas production, and possible inhibitions on anaerobic process. In that direction, different amounts of solid wastes were used for the purpose of loading the reactor.
The results obtained can be summarized as follows.
Anaerobic treatment of solid waste is a very sensitive process on changing the loading regime. That is why this process takes a lot of time (at least 8 months) to achieve maximum loading and efficiency. In the same time, possible over loadings can vary causing fast negative impacts on whole process, with very long period for recovery, but sometimes without any possibilities to go back in steady state.
Process imbalance in an anaerobic digester will normally lead to accumulation of VFA resulting in decrease in pH. The increase in acid concentration may not be immediately registered as a drop in pH if the buffer capacity of the material in the reactor is high. The organic acid accumulation therefore has to reach a high level before it is detected as a drop in pH. At that point the organic acids significantly inhibit the process. pH is easy and inexpensive to measure, and it is effective indicator in systems with low buffer capacity. In any case, measurement of VFA together with alkalinity is much better for the purpose of control and to avoid VFA inhibitions.
Demonstrated treatment requires continuous supervision, due to the need to install on-line measurement of: loading quantity, biogas quantity, biogas quality (at least content of methane), effluent quantity, temperature inside of reactor, outside temperature. In addition it is necessary to analyse pH, alkalinity and VFA on daily basis.
Continuous loading of pilot plant with smaller quantities of waste would be preferable comparing to the loading of once per day and decreases sensitivity of the system
Similar results on samples from three different sampling points suggesting that pilot plant was not a real plug flow reactor, but more like CSTR reactor. Mixture inside was more liquid then slurry.
By the end of test period, it can be considered that pilot plant was still at start-up period. Hydraulic retention time decreased from 135 days down to 54, and in overloading period reached the value of 27 days.
Thorough knowledge on agro-industrial wastewater characterization and anaerobic digestion process was an important base to generate treatment scenarios for several agroindustrial wastewaters.
The objective of design was to obtain the effluent quality as prescribed by the EU Directive 271 from 1991 on Urban Waste Water Treatment. The design was worked out and described in a more detailed way for brewery wastewater. The calculations were also performed for wastewater from the sugar, potato and fruit and vegetable processing factories. For all types of wastewater anaerobic digestion in a high rate UASB (Upflow Anaerobic Sludge Blanket)reactor was selected as a core technology. Further, the whole treatment scheme consisted of a pre-treatment (screening, buffer tank and plate clarifier) main treatment and post-treatment. With the main treatment in the UASB 90% of the COD is removed. When using the UASB reactor, post-treatment is needed to remove the residual solids and COD as well as nutrients.
The anaerobic reactor is more feasible than an aerobic treatment system as the required reactor volume is smaller and less excess sludge is produced.
Using an EGSB (Expanded Granular Sludge Blanket)reactor instead of an UASB reactor could also be feasible. The volume and costs for this reactor are lower than these of the UASB reactor. However more suspended solids will need to be removed in the pre-treatment as the hydraulic residence time in the EGSB reactor is much lower and less solids can be removed in the reactor. The EGSB reactor is also more complex in construction. Using an anaerobic treatment system to treat the brewery effluent is innovative for the Western Balkan. Since there is still a little experience regarding design, maintenance and operation a somewhat simpler UASB is selected in the first instance.
The wastewater entering the reactor does not need to be heated. This would cost energy and the need to install heating equipment while the profit in terms of smaller volume needed is very small as the organic load already is very low at 25oC.
The proposed treatment design can be implemented to reach the EU requirements regarding the effluent from agro-industrial industries. However more factors have to be considered like financing, available space, attitude of the factory, local regulations.
It is finally recommended to analyse the whole production process to come to the most sustainable solution. There are several points where the process water consumption can be lowered. Possibilities for water reuse should be considered. With a smaller effluent (=influent to the treatment) flow, the required reactor volume is also smaller. This would especially be interesting for breweries with a larger daily flow than the one examined in this study.
Also the possibilities of the collection of solid waste residues (e.g. for brewery yeast residues and spent grains) during the production process should be looked at. This would result in less suspended solids in the wastewater and therefore less removal needed during the pre-treatment. Some solid waste residues separately collected during the process can be reused and additional profit can be gained (e.g. yeast and spent grains can serve as cattle feed).
The objectives were to determine the possibilities for using stabilised sludge, obtained from the anaerobic treatment of solid waste from the agro-industry (brewery, sugar factory and edible-oil refinery), in agriculture and forestry, to fertilize and/or condition the soil.
For the investigation into the possibilities of applying anaerobically stabilised sludge, and anaerobically stabilised sludge post-stabilised by the addition of CaO from the chosen agro-industries, humus-poor soil of acidic character was obtained from a private farm near Novi Sad. Soil with these characteristics was chosen for investigation based on the fact that in Vojvodina (northern province of Serbia), the total area of this soil type is 1412 ha. pH was taken as the key factor in the evaluation of the maximum amount of the investigated sludges which can be applied to the soil chosen for the experiment, considering the alkaline characteristics of the sludge.
By simulating rainfalls during the experiment, significant washing-out effects were observed on the nutrient content (above all nitrogenous matter) of the investigated soils. Because of its alkaline characteristics (pH 7.45-12.78), all of investigated anaerobic sludges (with and without post-stabilisation by CaO addition), can be successfully applied as a soil conditioner (the addition of 2.8-6.7% sludge to the soil results in a pH correction of 1.24-2.21 pH units in the soil).
The effects of the application of this sludges as a fertilizer resulted in an increase in nitrogenous matter content by 6-23% and phosphorous by 38-25%, depending of the applied slude on soil, and also leads to a reduction in the washing-out of these compounds during heavy rainfall. Besides that, based on the C/N ratio (which decreases), this sludge leads to soil stabilisation from a nutritional content aspect.
Comparison of the results for the application of anaerobically stablised sludge as a soil conditoiner and fertilizer, with respect to its effect on the pH of the soil and the content of useful materials, shows that all the investigated sludges are efficient pH correctors, but the highest-quality sludge in respect to increasing nitrogen matter content in soil was the sludge obtained by digesting solid brewery waste, and from the aspect of phosphoruos matter content the sludge obtained from edible oil refinery waste.
As fertilizers, slugdes post-stabilised by adding CaO increase the nutrient content and nutrient retention of the soil, but application of post-stabilised edible oil refinery slude is limited by its reduction of the water permeability of the soil, due to its quagmire- inducing effects.
In the agro-industry a large number of unit processes generate wastewater, from single processes or after mixing streams from several different processes. In addition, these wastewaters vary strongly in composition and concentration of pollutants, not only between different regions and agro-industries, but also within individual industries. For the realization of a cost-effective and sustainable management of these wastewaters reliable treatment techniques and safe re-use strategies need to be implemented, despite all the variations occurring in the wastewaters. To achieve this goal it is imperative that adequate measurement techniques are established, in order to characterize the wastewater composition and concentration. On the basis of this characterization the best available treatment technique can be determined. For this purpose on-line (semi-) continuous measurements are preferred because they allow efficient monitoring and optimal (automatic) control of the treatment process.
This work resulted in deliverable consisting of two parts:
- A review report on characteristics affecting anaerobic treatment and
- A review report on-line (semi)-continuous measuring techniques for agro-industry wastewaters.
Generally, agro-industrial wastewaters represent high concentrations of COD with high fractions of biodegradable material, making anaerobic treatment technology the pertinent approach to reduce the organic load and recover valuable resources.
However, as anaerobic treatment technologies are vulnerable to large influent variations and certain harmful components that may be present in argo-industrial effluents, special care must be taken when these technologies are applied. For the optimal operation of anaerobic wastewater treatment plants a number of measures exist to deal with these factors if wastewater characteristics can be measured on-line.
Candidates for on-line measurements are the parameters that are reported most frequently in literature on agro-industrial wastewater: COD, pH, Ntot, SS and BOD5, although rarely reported parameters that may have a large effect on the process, such as alkalinity, deserve more attention as an on-line parameter. Measuring principles for wastewater characteristics rely on a limited number of physical, chemical or biological techniques, or combinations of these.
The most important include: chromatography, electrochemistry, spectrometry and titrimetry. Some wastewater parameters can be assessed by several of these measuring principles. Several data processing methods exist to cope with the vast amount of data that is generated when several in-line instruments are continuously in operation.
Both parts of report constitute a comprehensive and detailed manual of practice to be used within and outside the consortium.
Anaerobic stabilization as a form of biological treatment is used to remove organic contaminants from the industrial waste streams. Studies of the conditions of anaerobic treatment of solid wastes from agro-industry (brewery, sugar factory and edible oil refinery) and its further stabilization on the increase in content of useful materials and simultaneous increase in sludge dewaterability (as an important characteristic from the aspect of its handling and utilization), was carried out on a pilot-plant scale.
Composition of the initial mixture in the experiments involving solid waste from brewery, sugar factory and edible oil refinery was chosen on the basis of the production characteristics of these industries. Studies of the effect of treatment of agro-industry solid waste and its further stabilization by adding CaO on the quality of obtained sludge in respect of content of organic matter and nutrients (N and P), as well as its dewaterablity, encompassed eight series of experiments with different substrates (two from brewery, two from sugar factory, and one from edible oil refinery). In the frame of the study of the effect of conditions of anaerobic treatment on dewaterability of the obtained sludge, the effect of pH correction of anaerobically stabilized sludge was also examined.
The results obtained can be summarized as follows: careful choice of the initial mixture composition can to a great extent influence the quality of the final product of anaerobic digestion in respect of content of useful materials and dewaterability of the sludge. However, for brewery solid waste composition didn't affect efficiency of dewatering. Anaerobic treatment of solid waste from edible oil refinery gave a sludge of increased nutritive value, but characterized by a long filtration time and a low efficiency of dewaterability.
The comparison of the results of anaerobic treatment of different substrates in respect of content of useful materials and dewaterability shows that the highest-quality sludge in respect of nitrogen matter content was obtained by digesting solid brewery waste (C/N ratio 5.39 and 10.85), and from the aspect of dewaterability by digesting sugar factory waste. Adjustment of the pH of the investigated anaerobic sludges to pH≤3 yielded improved dewaterability.
Stabilization with CaO of the anaerobic sludge originated from brewery solid waste reduced content of nitrogen matter, important for its potential use as a fertilizer, but on the other hand increased the dewaterability of the stabilized sludge. Stabilization with CaO (6-10%) of the anaerobic sludge originated from sugar factory waste yielded a decrease in organic matter content, whereby the C/N ratio showed also a decrease. Judging from dewaterability parameters, a dose of 6% was sufficient to improve the sludge quality. Stabilization with CaO (4-10%) of the anaerobic sludge originated from edible oil refinery yielded improved sludge characteristics in respect of content of useful materials and dewaterability, whereby a dose of CaO of 4% was sufficient to produce the desired positive effect.
There are water pollution problems due to inadequate agricultural practices in the CEE countries by intensive use of fertilisers and pesticides. Since there has been a huge load of pollution led into ecosystems, he demands implementation plants for water quality management. Resident fertilisers and pesticides on the agricultural products could be arriving in the food chain or the off water of agro-industries.
Several methods for the economic and ecological evaluation of processes are introduced in literature. Among them Life Cycle Assessment (LCA), Life Cycle Costing (LCC) methods as well as input-output analysis, social matrices, change analysis and many other, Some of them can be calculated mathematically, some of them are argumentative. Mixtures of methods are always possible.
Common to all techniques is that a more or less complex model of the process, the streams inclusive up- and down-streams and the plant has to be made with respect to different scenarios of possible processing options. After doing so, the environmental pressures (e.g. CO2 or energy) should be determined.
One of the methods, the Simple Additive Weighting (SAW) method is viewed at. The ecological impact in was focused the green-house-effect and the energy-consumption in terms of carbon dioxide emissions. The economical impact was considered to be dependent on the investment and the costs for energy. The factors given for the evaluation are the emissions in the sense of aggravating green-house-effect, the energy consumption and the ability to save process water. The evaluation of the impacts was done with respect of the process itself. It was shown, that the reverse osmosis uses less energy than the ozonation, but the retentate has to be disposed.
A calculation of the energy consumption and the transport of retentate for disposal in terms of CO2 emission showed, that the use of ozone for polishing the effluent of the model plant emits 65 % less carbon dioxide than the treatment by RO. The reuse of the water has to be viewed at from the point of official regulations on site and the purpose, the water should be used for. For example, water for cleaning-up purposes and for flushing procedures or for transportation of the fruits can be reused at a higher level of contamination than water for the direct contact with food production depending on the specific regulations.
The emission of carbon dioxide (or equivalents) is only viewed from the point of additional production. Thus not regarding the emissions of the treatment process, since the overall balance of such processes is zero, because all CO2 is produced with natural non-fossil resources. The overall energy production of a lagoon is zero whereas the energy production of other processes as UASB and the investigated plant could reach from 297 to 1,471 MWh/a for the model treatment plant, depending on the type of waste or wastewater and the treatment process. Regarding the waste, it is important to consider the space needed for disposal in landfills or the reusability in the agriculture.
Certain types of organic wastes can be used as fertiliser, if they have a high organic content and they can be used for improving the quality of the soil. The eutrophication of the waste or wastewater was expressed as output of nitrogen from the processes. The results show that the raw water (sugar factory) as well as the effluent of an UASB reactor does not diminish the oxygen demand of the water. If a MBR-system were attached, the oxygen demand goes towards 0. If a lagoon was used for water treatment, there would be still a non-neclectable oxygen demand. Additionally, the emissions of a lagoon cannot be specified since the processes happen parallel and simultaneously.
Moreover, an energy recovery or the production of green energy is not possible by a lagoon. In contrast the reduction of nitrogen, a German plant produces products as fertilisers and proteins, which can be sold. The calculation of the overall costs shows, that, if only the RO and the ozonation are regarded, the costs for the ozonation are 61 % of the costs for the ozonation. But is the whole process including the anaerobic treatment and the MBR into account, the option using the RO is only 75 % of the costs for the option with the ozonation. The degree of water-reclamation depends on the intended purpose and the local regulations. The consequence is, that if the water is needed for flushing no polishing could be of need.
The use of water for irrigation as well depends no the matrix of the waste or water, since there are influences to the soil or groundwater aquifer and has to be evaluated for a specific need. A comprehensive definition of technology should be done with a holistic approach instead with a fixed set of criteria. This is to be done with concepts and sets of definitions, which agree with the site-specific circumstances especially, when the irrigation of wastewater or treated wastewater is considered for the reuse of water.
Processes in the agro-industry need much process water for eg. cleaning processes. This has to be considered, when regarding an integrated process management. Waters of such industries are highly loaded with organic contents, which lead to extensive eutrophication if discharged uncleaned into ecosystems. They undergo (e.g. in Germany) several processes for cleaning, reuse of these substances in the water and for disposal or recycling.
An example is the water treatment process in the potato-industry, which has been used for this work. The excess water of this process is treated by several processes to use the contents of this water for production of fertiliser and energy. After these steps, the water has to be cleaned biologically to reach the limits of official regulations for discharge, such as the concentration of COD, nitrogen concentrations or CFU. Since the biological processes only can alter the biological degradable contents of the waster water, recalcitrant contents have to be treated in a different way.
The regulations and needs for the excess water differ strongly from the regulations and needs for process waters. Process water in the agro-industry has to be useable according to regulations for food processing. This means eg. nearly germ-free and free of harmful contents for the human organism. So it has to be distinguished between the discharge of excess water to municipal receiving system and the reuse of excess water for the process itself.
Commonly, the last step of such water treatment processes is the reverse osmosis (RO) to retain contents like recalcitrant COD, salt or germs. Ozonation is a well-known application for drinking-water disinfection. It is a conventional treatment process for treating difficult wastewaters as landfill-leachate in order to remove COD among other contents, too.
Thus, it should be possible to treat excess water from the agro-industry to enable a reuse of the water or at least to enable a discharge into further treatment processes such as municipal wastewater treatment. But since the production of ozone is expensive and energy intensive, enhancements of this technique is needed. And the ozonation of wastewater has to compete with techniques as the RO, UV-disinfection or common processes as chlorination. The wastewater examined was taken from a MBR, treating effluent water from the potato-industry.
The process and fruit water is processed through several unit operation steps, which produce energy, fertiliser and proteins. The remaining water is treated by a membrane bio-reactor (MBR) with micro filtration for inlet into the municipal receiving system. A partial stream is processed via reverse osmosis for industrial water recycle. The volumetric flow rate of the water is 55 cbm/h, during campaign approx 110 cbm/h with a COD of 250 - 300 mg/l, (This is less during campaign due to the higher flow rate). The concentrations of ammonia and nitrate are 1-2 mg/l resp. 5-6 mg/l. The samples for the tests of the ozonation efficiency are taken between the MBR and the RO.
The wastewater was analysed regarding the dissolved COD and the CFU. Measurements of TS or VSS have not been done, because the micro filtration is capable to hold back the micro organisms in the biological treatment process. Samples of 250 mL were taken from the ozonsation-reactor prior to the ozonation and several times during the ozonation itself. The ozonation of the water containing 1.500 mg/L COD showed a degradation of COD of 20%. By adding H2O2, the degradation rate was improved up to 70% after one hour of operation. The number of CFU could be eliminated after 3 minutes of ozonation. Summarising, it can be stated, that disinfection ad degradation of recalcitrant substances for the reuse of wastewater at least in the potato industry is possible by polishing the effluents of a MBR by oxidation processes. If this can be done by ozone itself, or if an additional activation of the ozone and the water to a radical reaction system (AOP) is needed has to be shown regarding the specific wastewater, which the application is to be designed for and the official regulations for process water in the specific industry on site.
Moreover, the use of the processed water as water for the direct food-production of the use as washing water has to be focused. Additionally, the efficiency of the post-ozonation of wastewater with other treatment processes as eg. reverse osmosis has to be viewed at regarding economic and environmental impacts on site.
The aim of the experiments carried out on mixture of brewery wastes was to improve organic matter hydrolysis by various physico-chemical and biological treatment procedures. Starting material (defined mixture composed of 25, 3, 7 and 3 parts of spent grain, hot break and spent hops, spent secondary yeast, and spent kieselguhr, respectively) was chosen for investigation on the basis of the expected production and use part of solid wastes from breweries for animal feeding.
Experiments were carried out under laboratory conditions in order to establish conditions yielding most effective hydrolysis. In view of chemical composition of solid waste from brewery (native substrate), which is characterized by a high content of proteins cellulose, nitrogen-free extractable matter, etc. the following types of hydrolysis were performed: biological hydrolysis (yeast thermolysates, rumen) and chemical hydrolysis (NaOH, H2O2).
The possibility of hydrolyzing native mixture exists, but it is a long-lasting process (about 30 days). For enriched substrate results were obtained only for COD, but it was concluded that the problems in hydrolysis are not expected with the increase of spent grain portion. For 5% of rumen added to substrate, slight acceleration of the hydrolysis process occurred and for 20% of rumen the problem with biological contamination of samples was encountered. Inhibition by ammonia is highly possible, methane-formation process very likely to occur very quickly, and results were of a poorer quality than with 5% of rumen (high ammonia, high pH).
Regarding yeast addition, a slight acceleration of hydrolysis was observed compared to control samples. Results for 450C/60 min and 650C/45 min of yeast themolysate addition were of a comparable quality. Treatment with NaOH solution improved the hydrolysis. With H2O2 addition maximum value for COD appeared earlier than in case where only NaOH was applied but it was somewhat lower value. Results for mixtures with addition of yeast thermolysate upon hydroxide treatment were not as good as the ones obtained by using anaerobic sludge as inoculum.
On the basis of characterization of the investigated hydrolytic processes under different conditions (with and without biological and chemical pretreatment) at the beginning and at the end of the experiment it was concluded that best results were obtained in the pretreatment with NaOH solution (fastest hydrolysis and the lowest C/N ratio).
Usually, wastewater from agricultural industries is treated with a biological treatment plant. Conventionally, this implies the use of a post settlement basin. Replacing this by a membrane system (MBR) will increase the concentration of dry substance and can lead to higher con-centrations of micro organisms, which will improve the cleaning capabilities of the biological system significantly. The decrease of the effectiveness of the membrane is induced by fouling and scaling. This leads to a significant amount of chemicals and operation time needed for cleaning and maintenance of the membranes. A new, practically not tested possibility of reducing the fouling and scaling is the use of ultrasound (US) to clean the membranes in-situ.
Different membranes and different US-intensities have been tested with test substances to measure the flux-rate with respect to the pressure. Tests with membrane of the type C030F and UF-PS-100H with demin. Water and without ultrasound did not confirm the technical data given by the manufacturer after two and four hours of operation. PVP K30 was rejected at 80-88 % from the membrane C030F and at 10 % for the membrane UF-PS-100. For further investigations, the membrane UF-PS-100 was chosen with the use of ultrasound at 15.5 kHz. It has been found, that the flux-rate increases by using ultrasound. Nevertheless, after approx. nine hours, the membranes surface has to be cleaned conventionally. The rejection of the membrane could be increased by ultrasound as well. No damage of the membrane was found during the tests. With these results a pilot scale membrane module with ultrasound has been constructed and built up. Further tests with the pilot scale module have been made.
Following the results of these tests, small modules and an MBR-system have been built additionally for investigations. It was found, that the pure water flux of the membrane increases with the ultrasound intensity but is compensated by high temperatures. Tests with dextran showed no or little rejection of the hollow-fibre membrane even at the cut-off of the membrane, which was a hint of damage to the membrane. The damage of the membrane was confirmed by the particle size distribution of a kaolin suspension in permeate and feed, because no differences between the distributions were found. Pre-tests of the new small module with approx. 0.1 m2 total membrane surface showed volumetric flow rates of 2,500 mL/h, which confirms the data given by the manufacturer. The combined treatment of wastewater with activated sludge followed by filtration through the membrane module resulted in decreases in the organic material content in total of 89-95%, and in the biodegradable organic fraction of 88-97%, with respect to HRT (1.13-2.3 days). The contribution of the activated sludge to the total organic material content was in agreement with previous investigations, and lay in the range 45-79%. The efficiency of the membrane process alone, in reducing this material, was in the range of 63-81% with respect to their content in water after activated sludge treatment, and depending on the applied organic load. Permeate show a slight fluctuation under the investigated organic load conditions, suggesting a high efficiency of the membrane process in decreasing the organic content retained in the water after activated sludge treatment. In terms of ammonia content, with the activated sludge treatment, a high removal efficiency of 93-96% was achieved, to values <1 mg NH4+/l. Further treatment of the wastewater using membrane technology decreased the ammonia content further (<0.6 mg NH4+/l).
Since water contents can react with dissolved ozone, one major task is to dissolve the ozone in the liquid. Commonly this is done with injector for the gas and a booster-pump for the liquid. The advantage of such a system is a fine bubble-size distribution and therefore a good mass-transfer for the ozone. The disadvantage is the high energy input needed.
The new system consists of a stirred bubble column reactor with installations to improve the back-mixing behaviour. The ozone intake is realised with a ceramic diffuser with small pore sizes to enable a small bubble-size distribution with a very low energy intake. The installations divide the reactor in 4 parts. The volume can be divided into volumes of 7.5 L, 11.0 L and 14.5 L and is operated as cascaded bubble column. The concentration of ozone in the gas was measured with a ozone HC-500 by PCI-Wedeco. The reactor was equipped with a bypass for the gas, thus making it possible to measure the ozone concentration in the inlet to the reactor as well as in the off-gas from the reactor. The ozone was produced with a generator SWO 100 by WEDECO from technical oxygen.
During the tests, the ozone production of the generator was adjusted via measurement in the bypass of the reactor prior to switching the gas for inlet into the reactor and measuring the off-gas. The amount of ozone transferred into the liquid was controlled by the time of ozonation.
The residence time distribution was measured with the HYDROMES-System made by the institute for thermal process-engineering of the Technical University of Clausthal. This device can measure the concentration of a tracer as differences of conductivity. The difference of the conductivity is directly proportional to the concentration of a salt-tracer. For the tests, 100mL of a salt solution was applied to the inlet of the reactor as a pulse mark during the continuous operation of the reactor.
The reactor was operated with several flow rates of demineralised water and air. The investigation of the residence time distribution has shown a strong influence of the stirrer and the volumetric flow rate of the fluid into the reactor. A strong influence of the gas flow rate could not be found. By varying the active volume of the reactor, it can be adapted to different needs. Compared with a CSTR, the residence time distribution showed an improved back-mixing behaviour with all tested reactor volumes.
Anaerobic treatment technologies are used throughout the world for the effective treatment of municipal wastewaters and sludges as well as for the treatment of a wide variety of industrial wastewaters and organic solid wastes. This technology is particularly attractive in developing countries since the energy required for operating the process is minimal compared to the energy required for aerobic processes. In addition, energy for on-site use is produced in the form of methane gas, which can be converted to mechanical or electrical energy for local use. Moreover, effluent from anaerobic plants can have high fertilizing value and can be successfully used for land conditioning.
Solid wastes from agriculture, farming and food processing industries due to the high organic contents and biodegradability are good candidates for undergoes anaerobic degradation.
Unfortunately, the comparison of literature research data and drawing of conclusions is difficult because the diversity of reactor designs is matched by a large variability of the treated waste and choice of operational parameters (retention time, solids content, mixing, recirculation, inoculation, number of stages, temperature, ...).
Under this conditions there certainly does not exist a consensus over the optimal reactor design to treat agro-industry solid wastes. The reason most likely lies in the wide range of different waste sources, complexity of the biochemical pathways involved and the relative novelty of the technology.
The results presented contribute to asses the most promising technology for the anaerobic treatment of agro-industries solid wastes and, based on data obtained from the laboratory scale to provide data for process scale-up and operation at full scale. The chosen feedstock consist of wastes from the potato processing industry.
Moreover, the ADM1 model for anaerobic digestion has been validated and shown to be able to reproduce most of the measured parameters.
The experiments were carried out in tree different types of laboratory-scale reactors: conventional 5l CSTR reactor, inclined plug-flow reactor and a two-stages reactor consisting of two vessels operated in series, 6l acidogenic vessel with separation of solid and liquid phase and 6 l methanogenic vessel with variable useful volume depending on the amount and quality of liquid phase separated in the acidogenic reactor. The reactors were constructed of Plexiglas and placed in thermostatic chamber at 35oC together with gas measurement devices for each reactor.
All the reactors were equipped with a glass bottle for gas sampling just after gas cumulative volume measuring devices. Each sampling devices was protected from air intake by means of an hydraulic seal.
The reactors were fed once per day and five days per week thus the effective organic load has been calculated on a weekly average base. The performance of the reactor was tested in response to different loading rates (0.5-4.5g COD/L/day) and operational regimes.
Cumulative gas production has been recorded daily whereas characterization of the effluents has been normally performed weekly or every time was deemed interesting (i.e. change on organic load, gas production).
The Anaerobic Digestion Model 1 (implemented in the AQUASIM software for simulation of aquatic systems) has been adopted and after calibration using data from lab. scale reactors validated using preliminary data from a pilot scale inclined plug flow reactor.
Inclined plug flow reactor seems to be a good compromise between performances and simplicity of design and operation in treating agroindustrial wastes characterized by high biodegradability and high moisture content.
Its performances can be compared with Continuous-flow Stirred-Tank Reactors while whole plant design and operation are slightly easier.
Moreover, possibility that some settling occur in the upper part of the reactor can increase effluent quality in terms of suspended COD.
Mathematical model ADM1 prove to be able to simulate the process with a good accuracy. Further refining of the model could be able to predict overloading situations and to predict maximum achievable OLR.
From experimental data, operation at OLR up to 5 kgCOD/m3/d can be achieved. However, other problems than biological stability (i.e. foam) can be expected at high load (greater than 4.5 kg/COD/m3/d).
Based on characterization of the agro-industrial wastewaters, the wastewater from the potato industry Marbo, Backi Maglic, Serbia and Montenegro, was selected as the most suitable industry for the pilot plant investigation. UASB pilot plant reactor was dimensioned based on the wastewater characteristics and the possibilities for location of the plant in the production part of the factory.
Reactor volume, calculated based on the hydraulic loading rate, is 0.53 m3 with the height of 5.5 m giving upflow velocity of 0.5 m/h. The reactor is equipped with six sampling points and a gas-solid-liquid separation. The reactor content is heated to ca 30oC. All produced methane is collected and its volume is measured. The seed sludge should have adequate settling velocity, methanogenic activity, as well as the capability of fast adaptation (during 2-4 weeks).
As the seed sludge, the granular anaerobic sludge, developed in the IWB laboratory was used. A number of typical parameters to control reactor performance are regularly monitored (COD, VFA, pH, alkalinity, methane, influent flow, nitrogen and SS).
Pretreatments are largely used to make substrate better accessible for the biological digestion and thus optimising the methanogenic potential of the waste.
Seven different type of mechanical, chemical and biochemical pre-treatments have been tested to enhance the anaerobic biodegradability of selected agro-industrial wastes. Wide spread use of these techniques for pretreatment of waste as well as their simplicity were the main selection criteria.
The experiments were carried at the laboratory scale using Biochemical Methane Potential (BMP) test that is widely used for measurement of substrate biodegradability.
The results obtained can be summarized as follows:
Waste from potato processing industry has characteristics of readily biodegradable substrate contributed to high starch content that is easily dissolved in the mixed liquor.
The data obtained indicate that pre-treatments applied had no significant effect on biodegradability of potato waste in terms of total amount of gas produced.
Waste from brewery industry has characteristics of hardly biodegradable wastes. All pretreatments applied had positive impact on initial gas production in first five days compared to the control sample, while the highest impact on total gas production had combination of alkaline pretreatment with high temperature and acid pretreatment.
Waste from sunflower-oil industry also has characteristics of hardly biodegradable waste that is converted only 25% to methane if no pretreatment is applied. The highest impact has been recorded using enzyme Novozym, converting COD 100% to methane in 18 days.
The feasibility of ananerobic membrane bioreactors (AnMBR) for the treatment of wastewaters from agroindustrial proceses was assessed. Experiments were performed at mesophilic and thermophilic conditions using three 3.7 L useful volume Anaerobic Membrane Bioreactors (AnMBR) equipped with submerged polysulphone micro-filtration membranes (Triqua, The Netherlands).
An emphasis was put into membranes performance, since this represents one of the most important cost in membrane bioreactors for wastewater treatment. In a first stage, the effect of flux, gas sparging, solids concentration and temperature on the critical flux was evaluated. Critical flux concept is used to define flux at which noticeable cake layer formation begins to take place. Cake formation rates over critical flux and its reversibility were also studied. In a second stage, long term operation of MBR reactors was studied, both under mesophilic and thermophilic conditions. Finally, the presence of non-acidified organic matter was studied under thermophilic conditions.
Biomass concentration showed to be the most important factor for cake formation in mesophilic MBRs. An increase in solids concentration reduce the critical flux and increases cake layer formation rate due to an increase of convective flow of particles towards the membrane. Gas flow rate has the opposite effect, increasing critical flux and decreasing cake formation. Thermophilic operation reduced drastically the effect of operational conditions, in comparison with mesophilic operation. Long term operation showed an important decrease in critical flux, probably due to a reduction in particle size distribution.
This may have been the result of cell lysis products that accumulate in the reactor. Cake formation showed to be reversible under mesophilic and thermophilic conditions, when studied in short term experiments. Nevertheless, on long term operation cake consolidation may occur, especially under mesophilic conditions. This cake layer cannot be removed by back-flush cycles, and a physical cleaning is required.
Presence of non-acidified organic matter induced the development of a high amount suspended acidogenic bacteria. This produced an increase in suspension viscosity and a decrease in particle size, which a consequent decrease in critical flux.
Anaerobic submerged MBRs economical feasibility is restricted by low critical fluxes attained in this study. Due to fast cake formation rates over critical flux, operational flux is likely to be restricted to a range close to critical value. Low levels of irreversible fouling were observed, with the exception of cake consolidation, as already mentioned. This means that the increase in shear forces over membrane surface should be efficient in increasing levels of critical flux. In an external membrane configuration, hydraulics can be more easily controlled. Further research in that direction would be of interest. Anyhow attention has to be set to the eventual negative effect of extreme shear forces over bacteria viability.
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