Development of pilot scale nanofiltration system for yeast industry wastewater treatment
© Rahimpour et al.; licensee BioMed Central Ltd. 2014
Received: 17 April 2013
Accepted: 26 February 2014
Published: 4 March 2014
The treatment of the yeast industry wastewater was investigated by nanofiltration (NF) membrane process on a pilot scale. Two wastewaters were used as feed: (i) dilute wastewater with COD 2000 mg/L and (ii) concentrate wastewater with COD 8000 mg/L. The permeate flux, COD retention, color and electrical conductivity (EC) removal were evaluated in relation to trans-membrane pressure and long-term filtration. A linear growth in permeate flux was found with increasing in trans-membrane pressure for wastewaters. In addition, the COD retention, color and EC removal increased with trans-membrane pressure enhancement. The results obtained from the long-term nanofiltration of dilute wastewater indicated that the permeate flux decreased from 2300 L/day to 1250 L/day and COD retention increased from 86% to 92%. The quality of the permeate in term of COD is lower than the discharge standard in river (200 mg/L). Thus, this process is useful for treatment of wastewaters produced by yeast industry.
The increasing demand of bread as staple food of human beings has developed backer’s yeast industry. Baker’s yeast industry is an important and developing industry in Iran. In yeast industry, the sugar beet molasses are used as a main raw material. These molasses contains 45–50% residual sugars, 15–20% nonsugar organic substances, 10–15% ash (minerals), and about 20% water. During yeast fermentation, the sugars contained in the molasses are a source of carbon and energy. However, a major part of the non-sugar substances in the molasses are not assimilable by the yeast and are released unchanged to the processing wastewater. These compounds represent the principal waste from the yeast production process. A high chemical oxygen demand (COD), dark color, and high concentrations of total nitrogen (Ntot) and non-biodegradable organic pollutants are the characteristics of the wastewater produced by yeast industry. Most of the contaminants in the wastewater are due to the use of molasses as a main raw material. Two types of wastewater are produced in Iran's yeast industry; (i) concentrate wastewater with COD of 25000 mg/L; this wastewater is originated from yeast separators and processes such as centrifuges and rotary vacuum filters and (ii) dilute wastewater with COD about 3000 mg/L; this wastewater is created from floor washing and equipment cleaning. The concentrate wastewater is firstly treated by evaporation process. This treated wastewater is combined with dilute wastewater. The COD value of wastewater at this step is about 8000 mg/L. The combined COD is sent to the aerobic treatment stage. The minimum COD at the end of aerobic treatment is about 2000 mg/L. In Iran, each of yeast factories produces about 1000 m3/day of wastewater, which is mostly treated with an anaerobic biological process. The most of yeast factories in Iran has developed and improved their biological wastewater treatment process to reach effluent targets. However, the current technology is still not meeting the environmental requirements as the total treatment efficiency in terms of COD is only about 70–80%.
Successful applications of membrane process for the treatment of industrial wastewater can be found in the recent literature. In fact, membranes technologies provide an important solution in environmental fields such as pollution reduction and water reuse, recycling valuable components from the waste streams. Nanofiltration is a pressure driven membrane process which is somewhere between reverse osmosis and ultrafiltration. The major transport mechanism in nanofiltration membranes is solution-diffusion mechanism. These membranes contain fixed negatively charged functional groups on their surfaces. Both nanofiltration (NF) and reverse osmosis (RO) are good alternatives for wastewater treatment because the high reductions in the conductivity, COD and color can be obtained. Nevertheless, un-treated wastewaters can not be used directly for membrane processes because these have major influence on nanofiltration or reverse osmosis membranes. Therefore, it is necessary to carry out a very exhaustive pre-treatment in order to avoid membrane fouling and membrane deterioration[6, 7]. During membrane treatment of yeast wastewater, two flows are generated (i) Filtered stream (permeate) which can also be used as process water in the fermentation industry and (ii) concentrate stream containing high amounts of recalcitrant organics that must be disposed off. Usually, to avoid the discharge of concentrate stream in environment, the concentrate stream is recycled back to the biological treatment plant. Few researches have been carried out about the wastewater treatment of yeast industry, especially with membrane process[8–11]. The most pilot scale nanofiltration processes have been developed for treatment of textile industry wastewater[12–15]. The main objective of this work was to investigate the application of pilot plant of nanofiltration process for treatment of the yeast industry wastewater. The ability of this pilot plan to COD retention, color and conductivity removal of wastewater was investigated.
Characteristics of dilute and concentrate wastewaters
2000 ± 100
8000 ± 220
43 ± 5
180 ± 15
6400 ± 140
14000 ± 660
3200 ± 110
9880 ± 400
Nanofiltration membrane module
The NE4040-90 nanofiltration membrane module was used in pilot scale setup. This nanofiltration membrane module was purchased from CSM, Korea. The membrane module is in a spiral wound form and has a filtration area of 7.9 m2. This thin-film composite membrane was made from interfacial polymerization of trimesoyl chloride and m-phenylene diamine (TMC/mPDA) on polysulfone micro-porous membrane with a non-woven polyester backing. The rejection ranges for sodium chloride and magnesium sulfate are 85–95% and 99.5%, respectively (reported by manufacture). The maximum free chloride concentration is 0.1 mg/L. The maximum operating pressure and temperature are 40 bar and 45°C, respectively. The permitted operating pH range is 2–11. In addition, the maximum feed flow rate and minimum concentrate flow rate are 4 and 0.91 m3/hr, respectively.
Ultrafiltration membrane cartridge
Characteristics of hollow fiber ultrafiltration cartridge
Out-to-In hollow fiber
OD450 μm,ID3500 μm
Ventilation rate of N2
≥7.0 × 10–2 cm3/cm2.cmHg
Product water turbidity
≤ 0.2 NTU
SDI ≤ 3
TOC removing rate
20% ~ 50%
Due to the presence of suspended solids in the yeast industry wastewater, pre-treatment of the wastewater is necessary to prevent the NF membrane fouling, plugging and deterioration. This may lead to decline in flux and rejection or even membrane failure. Two stages were selected for initial pre-treatment of wastewaters; sand filter and cartridge depth micro-filter. The vessel of sand filter was filled with three type of silica; large, medium and fine particles, respectively. The cartridge depth filter is a polypropylene porous membrane with a pore size of 10 micron. After these two steps, the majority of large and medium particles were removed from the wastewaters. This is very important to prevent the ultrafiltration and nanofiltration membrane from damage.
Pilot scale nanofiltration setup
Results and discussions
Permeate flux of nanofiltration system during filtration of wastewaters
As shown in this figure, the permeate flux of nanofiltration membrane increased proportionally with increasing of the trans-membrane pressure. This confirmed that the operation was in the pressure-controlled region. As expected, flux increased linearly with increasing of trans-membrane pressure. Spiegler-Kedem Model can describe this behavior:
where Jv is the water flux, Lp is the pure water permeability, ΔP is the trans-membrane pressure (TMP), σ is the reflection factor of the membrane, and Δπ is the osmosis pressure. This equation shows the permeate flux (Jv) has linear relationship with trans-membrane pressure (ΔP).
The significant difference in permeate flux between two kinds of wastewater was observed. The permeate flux of nanofiltration system during filtration of dilute wastewater was higher than concentrate wastewater. The decline in permeate flux was nearly 51% for a change in the wastewater concentration from COD of 2000 mg/L to COD of 8000 mg/L. It is expected that the permeate flux would decrease for the concentrate wastewater rather than the dilute wastewater due to concentration polarization. Concentration polarization was obtained from the gradual increase of the boundary layer of highly concentrated solute on the membrane surface. This term is used to describe the accumulation of membrane-rejected solutes close to the membrane surface. As water passes through the membrane, the convective flow of solute to membrane surface is much higher than the diffusion of the solute back to the bulk feed solution. Consequently, the concentration polarization reduces the permeate flux of the membranes.
COD retention, color and EC removal
Long-term performance of pilot plan
The current study shows that the pilot scale nanofiltration process is technical and efficient process for treatment of the yeast industry wastewater. The permeate flux, COD retention, color and EC removal enhanced with increasing trans-membrane pressure during filtration of dilute and concentrate wastewaters. The long-term performance of pilot scale setup was investigated by filtration of dilute wastewater. The obtained results showed that the permeate flux decreased from 2300 L/day to 1250 L/day and the COD retention increased from 86% to 92%. It denoted that the COD of dilute wastewater decreased from 2000 mg/L to 160 mg/L. Therefore, the quality of the permeate in term of COD was lower than the discharge standard in river (200 mg/L).
The financial supports from Iran Department of Environment are gratefully acknowledged.
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