Question

For Pulp and Paper class:

1) Purposes of coating and calendaring?

2) Major differences between aerated stabilization basin and activated sludge process?

3) Primary sludge vs secondary sludge?

Answer 1

2)The ASB, also known as a lagoon system, is a large body of water used for biological treatment. The placement and continuous operation aeration equipment is both the most important part of the system and the most expensive. Untreated effluent comes into the primary clari­fier and then into the ASB. As the effluent passes through the system, the BOD or “food” present is used as energy and a carbon source for bacteria. Once the treated water has gone through the ASB it typically enters a polishing pond for tertiary treatment where the remaining TSS (total suspended solids) settles out before being discharged as the final effluent.

The activated sludge basin has a smaller overall footprint with a higher concentration of solids and active bacteria. It is the faster of the ASB. A secondary clarifier provides settling and recycles a portion of the settled solids (bacteria) back to the aerated basin (RAS) to aid in BOD reduction.

3) Primary Sludge: Sludge settled in primary settling tanks comes under this category which contains 3% to 7% solids out of which approximately 60% to 80% are organic. Primary sludge solids are usually gray in color, slimy, fairly coarse, and with highly obnoxious odors. This sludge is difficult to dewater without treatment, hence digestion is necessary. This type of sludge can be digested readily by aerobic or anaerobic bacteria under favorable operating conditions.
Secondary Sludge: This type of sludge from secondary settling tanks has commonly a brownish, flocculent appearance and an earthy odor. It consists mainly of microorganism containing 75% to 90% organic fraction and remaining inert materials.

1) The main purpose of calendering is to improve the surface properties of paper and to control the caliper profile. Compaction of paper surface and its structure improves the surface properties, the most important of which are smoothness and gloss.

The purpose of paper coating is to impart certain qualities to the paper, including weight, surface gloss, smoothness or reduced ink absorbency.