SNAP Alginate and Natural Products Ltd. situated at Ranipet, Tamil Nadu, India, is involved in the production of alginate of various grades from Sargassum sp. Their liquid effluent (generated at 30 kl per day) is highly acidic (around pH 1.4–1.8) due to the use of sulphuric acid as their process material. The Pollution Control Board regulations require that the acidic effluent is neutralized, and the industry was using caustic for that purpose. The solar ponds used for evaporating the effluent were converted to high rate algal (HRA) ponds and a pilot slope tank study was undertaken to shorten the duration of the pH increase, which was later scaled up.Preliminary laboratory experiments revealed that Chroococcus turgidus, a cyanophycean microalga, has the potential to increase pH and decrease total dissolved solids (TDS) and has a wide tolerance to pH and high salinity.
SNAP manufacture alginic acid and its various salts from the brown algae Sargassum and Turbinaria which are procured from the Gulf of Mannar, South India. In the industry, seaweeds are treated with mild acid for removal of unwanted salts and then treated with alkali for conversion of alginic acid into sodium alginate. The digested pulp is then diluted and settled and sodium alginate is recovered.
SNAP generates a solid and a liquid discharge. The solid discharge consists of the remains of seaweeds after the extraction of alginates which are composted. The liquid discharge is the wash water of the seaweeds, which is acidic and essentially contains organic TDS, sea salts, and sea sand.The spent water generated was divided in to three main categories: neutral, alkaline, and acid streams. The neutral water stream consists of wash water of various process equipments, tanks and floor, and the condensate water from the boiler. The alkaline water stream is produced during the digestion. The seaweeds are digested under alkaline conditions. The digested seaweeds are diluted with water and settled to remove the seaweed residue. The supernatant liquid from the settler is taken for further processing. The seaweed residue is further washed thoroughly and dewatered. The collected water is taken in the ‘alkali water recycle’ tank and reused for diluting the digested seaweeds. As for the acid water stream, the supernatant liquid from the settling tanks is acidified and precipitated to recover the product. The acidic water is collected in the acidic water recycle tank. A portion of the acidic water is neutralized and taken with the alkali recycle water and reused for diluting the digested seaweeds. A portion of the acidic water is reused for washing the seaweeds. About 30 kl of acidic water as a blow down is sent to the treatment plant. The acidic water was neutralized with caustic soda by the industry, settled to remove the total suspended solids and evaporated on solar evaporation ponds (SEPs). The dried solids were recovered from the SEP and stored in a secured landfill.
C. turgidus was isolated from SNAP ETP site and identified following the monograph of Desikachary28. As Chroococcus sp. can grow in both fresh water and salt media, it was first inoculated in Bold Basal medium and was then transferred to F/2 medium for maintenance. Laboratory cultivation was carried out at 24±1°C in a thermostatically controlled room and illuminated with cool white fluorescent lamps (Philips 40 W, cool daylight 6500k) at an intensity of 2000 lux in a 12:12 light dark regime.
The SEPs were built to evaporate the liquid effluent after neutralization. The total area covered was 11000 m2. The number of ponds built was 11. All the ponds were built in concrete and had a depth of 45 cm. Each pond is interconnected in such a way that the flow of effluent is by gradient. 30 kl of effluent is the input per day. To evaporate 30 kl of water per day at 4.5 mm per day the required area of the pond is 6750 m2. For our experimental purposes, Tank A (40 m×47.5 m×45 cm) was used. The area covered was 1900 m2 and the total capacity was 8850 l. The depth of the effluent was maintained below 40 cm. The SEPs were later termed HRA ponds when phycoremediation was employed. No mixing, agitation, or aeration was done.
The pilot scale sloping pond was constructed in reinforced cement concrete (RCC) and was designed with a dimension of 2 m (L)×2 m (W)×0.75 m (depth) with a sloping angle of the evaporating surface of 45°. The dimension of the sloping area was 4 m2. The holding capacity of the tank was 3000 l. The flow rate of the effluent was at 1800 l/h. 1 cm of water in the tank is equal to 40 l. The plant was run during the day for about 9.5 h.
The scaled-up sloping pond was constructed in RCC having a cross-sectional area of 1200 m2 (70 m×17.15 m parallelogram). The depth of the tank was 1 m and had a holding capacity of 1200 kl (Fig. 1). Pillars erected from the tank support the asbestos sheet slopped roof, angled at 45°. The slope angles from 240 cm to 165 cm for 23 m. This did not allow the rainwater to seep into the tank. The asbestos sheet was painted with black bitumen to improve the absorption of solar radiation.
Fig. 1 Schematic diagram of phycoremediation sloping scaled-up plant.
At the ground level (bottom surface of the tank) a slopped floor area adjacent to the sloping tank was built. This was also painted with black bitumen. The slope was 15 cm and had an area of 3000 m2 (70 m×43 m). The effluent enriched with the alga was allowed to flow by gravity from the sloping pond tank. At the highest point of the sloping pond the effluent was pumped and from the roof it flowed down and collected in the sloping pond tank again. The effluent was pumped with the help of a 7.5 hp motor, which consumed around 40 to 45 units of electricity per day.
The tank was of RCC with side walls measuring 70 m (W)×21 m (L)×1 m (D) with a thickness of 0.23 m. 120 concrete columns with a diameter of 22 cm were built in the tank of which 35 columns were on the sidewalls and the remaining 85 were in 5 rows of 17 each. The asbestos sheet was fastened to the columns with steel girders. The phycoremediation sloping tank faced in the east-west direction to maximize sun tracking. The tank had a capacity of 1200 m3 with a sloped area (roof) of 1200 m2 and a sloped floor area of 3000 m2. The total sloped area for evaporation was 4000 m2. The pump had a flow rate of 80 kl/h. On average 840 kl of effluent was maintained in the tank. Test bore wells of 22 cm diameter and 300 cm depth, three each on both sides of the phycoremediation sloping tank, were built to detect any seepage of effluent. The bore wells were periodically tested for any difference in TDS, chemical oxygen demand (COD), biological oxygen demand (BOD), and pH.
With the HRA pond, the desired pH increase occurred in 4–6 days. If the algal growth could be enhanced by better aeration and mixing, the time taken to increase the pH could be reduced. Aeration and agitation can improve the growth of algae and this could be achieved by circulating the effluent in a sloping pond. This could also increase the rate of evaporation. Hence it was decided to try sloping pond technology. Pilot plant sloping pond studies were undertaken.
The principle of the slopping design is to create a turbulent flow while the algal suspension flows through sloping surfaces. A pump returns the algal suspension from the lowest point to the top. The turbulence is produced by gravity. The flow speed of the liquid increases with the slope of the surface. Effluent enriched with C. turgidus was loaded into the slope pilot tank.
We first tried using a sloping surface made from galvanized iron (GI) sheet. The sheet was painted black so as to absorb as much heat as possible. The pH of the effluent was maintained in spite of adding 60 l of acidic effluent intermittently (Table 3). Evaporation was also significant at 40 l per day with the maximum temperature being 32°C. But the problem encountered with GI sheets was erosion and corrosion. The tip of the slope corroded and cracks in the paint occurred within a month. Hence we instead decided to use asbestos sheet coated with bitumen. A similar increase in pH and evaporation was achieved (Table 3). There was no corrosion and the bitumen also was stable. The temperature was measured between the white surface and the black surface of the sheet. A sloping surface of concrete coated with bitumen was also tried. The pH was stable and the evaporation was at 40 l per day.
Further studies were done to observe the change in pH and evaporation by increasing the number of circulations (by increasing the flow rate). The number of circulation was increased from 7 to 33 per day and, as a result, the evaporation increased from 20 l to 48 l per day. The pH of the effluent was stable in spite of addition of 60 l of acidic effluent. The number of circulations of the effluent enriched with C. turgidus did not have any effect on the pH. The effluent remained green and the cell density of C. turgidus remained constant and was around 3600×104 cells per ml.
After the successful completion of the pilot plant studies, it was decided to scale up the plant to 1000 times the capacity of the pilot plant. The pilot plant had a sloping area of 4 m2. It was decided to have a sloping area of 4000 m2 in two stages, one on the roof and the other on the floor (Fig. 1). With a holding capacity 1200 kl it can accommodate 40 days of effluent generation at 30 kl a day.
The plant was initiated with 720 l of effluent from a SEP, which was already enriched with C. turgidus. The effluent had an initial pH of 6.85. From day 1 onwards, the pH stabilized around 7 and the TDS was around 45000 mg/l. The average load of effluent was around 850 l and just one circulation was enough to evaporate 25 to 30 kl of effluent. Even at the end of 9 months, the data remained the same (Table 5). The effluent remained green and the count of Chroococcus remained constant and was around 4500×104 cells/ml. During rainy days the plant also served as a good rainwater-harvesting unit. Whenever there was rain, phycoremediation was stopped and the rainwater collected was sent to a water storage tank.Conventionally NaOH is used to neutralize acidic effluents. Chemical neutralization costs could be reduced significantly with an integrated iron oxidation and limestone neutralization process because limestone is less expensive than lime, and a highsolids-content sludge is produced30. Benassi et al31 suggested that chitosan microspheres could be used as an alternative approach for remediation of acidic coal mining wastewaters.
Encouraged by the pilot plant studies, a fully scaled up sloping phycoremediation plant was commissioned in September 2006. With the addition of around 30 kl of acidic effluent every day, the pH of the effluent remained constant at around 7.0 and the TDS stabilized at 49000 mg/l. There was no sludge formation even at the end of two years of operation (Table 1). Even after just one circulation of the effluent with a pumping rate of 80 kl per hour on the slopes, the desired evaporation of 30 kl was achieved. The level in the tank was kept around 850 kl against the tank capacity of 1200 kl. The sloping tank also served as a rainwater-harvesting unit during rainy days. It has been estimated that the amount of rainwater that could be harvested during a normal rainy season can supply 6 months water requirement for this industry. Presence of another blue-green alga, Oscillatoria only on the slopes of the sloping pond added to the efficiency of phycoremediation. The removal of Oscillatoria, which formed a thick scum over the sloping evaporating surface at the ground level, drastically reduced evaporation by 50%. Bioremediation removes the toxic pollutants once and for all. The pollutants may be bio-converted, biodegraded, or volatilized. Neumann et al39, established the fact that even metals like selenium are converted to volatile dimethylselinide by microalgae. Photoevaporation of bio-molecules by higher plants has been demonstrated with mass balance studies40. Luther10 has reported that the alga Scenedesmus obliquus was able to utilize naphthalenesulphonic acids as a source of sulphur for their biomass, releasing the carbon ring into the medium. Bio-utilization, bio-transformation, and bio-evaporation may explain the reason for TDS remaining stable at 49000 mg/l even after the addition of 30 kl of effluent per day with a TDS of 27600 mg/l. Our preliminary laboratory studies also revealed that C. turgidus reduced the levels of potassium, sodium, iron, chlorides, flourides, sulphates and TDS to a large extent22–24. So far, almost all the bioremediation technologies in treating acidic waters have been limited to the laboratory scale.
The first ever phycoremediation plant with C. turgidus has been working from September 2006 at SNAP industry very successfully. The plant evaporates 30 kl effluent every day without developing any sludge. pH correction and sludge reduction could be achieved without employing any chemicals. The industry is saving a substantial amount of money on chemicals and the environment is spared from the dumping of hazardous solid waste. We expect that phycoremediation technology could be employed to deal with other types of industrial effluents with similar success.
