Studies on effluents from paper manufacturing process shows that sulphite process produces the strongest waste and process using soda process produce the weakest waste. Since the pulp and paper manufacturing process is extremely water intensive, every attempt is made to reuse, recycle and recover water and chemicals from the manufacturing process.
- Water can be reused from soaking and debarking operations after marginal treatment such as settling and chlorination
- White water can be reused in showers of the foundriner and also used to dilute bleached pulp
- Steam used in drying can be condensed and reused
- Other important chemicals used in the process can be recovered.
Wastewater from pulp and paper industries can be treated by physical, chemical and biological methods depending on nature of wastewater and quality requirements of treated effluent before disposing in the environment.
The following techniques have been used for removal of colour, solids (suspended and dissolved - TSS & TDS), BOD and COD from the effluent.
- Colour in wastes from pulp and paper industries comes from wood extractives, lignin and its degradation products.
- Colour is mainly formed due to reaction of chlorine with lignin
- Activated Sludge process removes one third of the colour in waste by adsorption on sludge
- Coagulation by using alum, ferric chloride, lime and magnesium sulphate removes colour produces voluminous sludge
- Alum and clay can be used to remove 95% colour at a pH of 5.2
- Pyrolised charcoal can also be used to remove colour partially
- Calcium and other multivalent salts have also been used to remove colour
- A combination of alum, calcium hypochlorite and ferrous sulphate effectively removed colour, BOD and COD
- Several anionic, cationic and non-ionic polyelectrolytes in combination with alum, aluminium chloride, ferric chloride and magnesium chloride are used for removal of colour, suspended solids and COD
- Colour removal has also been reported by 'white rot fungi' along with addition of nutrients like carbon and nitrogen to the fungal culture
- Effluents from kraft pulp mills were decolorised by ultraviolet radiation in the presence of free titanium dioxide, free zinc oxide or zinc oxide supported on silica gel
The different biological wastewater treatment methods employed are:
- Activated Sludge Process
- Anaerobic treatment and
- Trickling filters
Aerated lagoons are frequently used for treatment of wastewater from pulp and paper manufacturing industries. In such cases, mechanical aerators are used for aeration and nutrient supplementation with Nitrogen and Phosphorus compounds is essential. Resin acids present in wastewater cause foaming resulting in reduced oxygen transfer. Proper spacing of aerators, adding anti-foaming agents or spraying a jet of water are measures against foaming.
As the wastes have a large amount of suspended solids, primary settling is provided prior to biological treatment to remove settle-able solids. Wastewater from integrated pulp and paper manufacturing industries can be modified by addition of Nitrogen and Phosphorus nutrients and made suitable for treatment by aerobic and anaerobic lagooning.
Activated Sludge Process can be used to reduce BOD of diluted wastewater. Nutrient supplements added are Ammonium Nitrate or Ammonium Phosphate.
Air can be introduced to produce increased turbulence and reduce the size of bacterial agglomeration. This technique can also be used to allow the biomass to withstand shock loads and sudden changes in pH.
Upflow Anaerobic Sludge Bioreactor (UASB) can be used to treat the wastes from pulp and paper mills. In this case, organic loading rate and Volatile Suspended Solids (VSS) were the factors interfering with treatment efficiency.
Lignin from pulp mill black liquor can be degraded by bacteria acclimated to lignin bearing wastewater using a biosorptive mechanism.
Studies for anaerobic-aerobic treatment of pulp mill effluents have demonstrated BOD and COD removals of 90% and 70% respectively. Effluent from the digesters can be treated in an Activated Sludge System and the overall BOD & COD removal of the system was 98% and 85% respectively.
An anaerobic rotating biological contactor has also been used to treat water from a waste paper based pulp and paper mill. Optimum conditions were influent pH = 6.9 to 7.0, temperature = 32 to 35 C, Hydraulic Retention Time (HRT) = 1 day, rotational speed = 10 RPM and
organic loading rate = 1.45 kg COD/m3/day.
Solid wastes generated in a pulp and paper mill can be used for:
- reburning of lime after desilication
- fly-ash in cement manufacture
- using lime sludge, fly-ash and effluent sludge as soil conditioner
- using bamboo and wood dust in fire bricks for energy generation
- the secondary sludge from waste treatment plant can be used for rearing fish
The treated effluent from a paper mill is fit for use as irrigation water.