How to know Effluent from Paper and Pulp Industries is safe?
Wood, fibre crops, or waste paper are all used to make pulp. It is made by breaking raw materials into fibres and then utilizing chemical, semi-chemical, or mechanical processes to make paper. The pulping process necessitates a large amount of water.
In terms of effluent discharge to the Environment, the pulp and paper sector trails only towns. A typical mill releases 80,000 to 1,30,000 cubic metres of wastewater into the water bodies each day. The pulp and paper business has long been regarded as a major consumer of natural resources (wood and water), as well as one of the most polluting industries in the world (air, watercourses and soil). The wastewater composition, and thus the effluent treatment efficiency and discharge characteristics to water, are highly reliant on the technology and raw materials used.
The four basic sections of a pulp mill are
(1) Handling of raw materials;
(2) A pulping line with a chemical and energy recovery system that is nearly closed;
(3) Using an open water system for bleaching, and
(4) The wastewater treatment system on the outside.
In which stage, the maximum effluent is created?
In the bleaching stage, up to 85% of the total effluent volume is created.
The degree of unbleached pulp delignification, the bleaching process, the washing loss, the type of wood, the desired end brightness, chemical and water consumption, and the degree of plant closure are all relevant indications of wastewater characteristics.
Pulp bleaching effluents contain a variety of organic and inorganic compounds in variable amounts. In most cases, organics make up one-third of the dissolved material, while inorganics make up the other two-thirds. The solid substance consists primarily of fibres, fibre fragments, and bleaching chemicals. Various species obtained from the raw material make up the dissolved organic matter, which is created during the pulping and bleaching process.
The effluent is a complex mixture of waste generated during the pulp and paper manufacturing process, comprising wastewater from debarking, pulp washing, bleaching, and cooking chemical regeneration.
Treatment methods in paper and pulp industry
A: Suspended particles are removed in primary treatment using clarifiers and/or settling basins. Bacteria break down biodegradable material and harmful components in secondary treatment, lowering biochemical oxygen demand, toxicity, and total suspended solid levels that can harm fish habitat downstream from the mill.
Activated sludge and aerated lagoons are the most common approaches.Filters and sequence reactors: Mobile Bed Bioreactor (MBBR) and Membrane Bioreactors—are two examples of these systems (MBR).
Anaerobic treatment is sometimes employed first, followed by an aerobic biological stage. Tertiary treatment is used in some mills to reduce toxicity, suspended particles, organics, and colour.
B: Wastewater is also treated in an aerated pond using a combination of physical, biological, and chemical processes. They have long residence durations, ranging from 3 to 20 days, and thus a considerable volume. They work with microbe concentrations of 100–300 mg/L (low solids concentration). Aeration devices (usually surface turbine aerators or bottom aerators) are used in these ponds to give oxygen to the wastewater and mix the contents of the pond, increasing microbiological activity.
However, due to low efficiency and the huge surface area required, the utilization of aerated lagoons has declined dramatically, with activated sludge systems accounting for 60–75% of all biological wastewater treatment plants in the pulp and paper sector. The aeration basin and the sedimentation basin are the two primary units of the activated sludge plant. The wastewater is treated in the aeration basin with a culture of microorganisms (activated sludge) that is present in high concentrations.
The retention time of activated sludge plants at pulp mills is around 15–48 hours.
In activated sludge systems, the solids concentration is typically 2000–6000 mg/L.A standard system's hydraulic residence time is 4–8 hours, while the cellular residence time (sludge age) is usually 5–15 days. For lengthy aeration, normal loads are between 0.05 and 0.1 kg BOD/kg sludge, and for low load processes, 0.1–0.3 kg BOD/kg sludge. The typical operating temperature is 35–37°C, with a dissolved oxygen content of 1.5–2.0 ppm.
The removal efficiencies obtained depend on the wastewater residence duration and operation parameters. BOD-5 removal efficiency range from 85 to 98 percent, and COD removal efficiencies range from 60 to 85 percent. TSS elimination efficiency utilizing primary and secondary treatment is estimated to be around 85–90%.
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