Sludge Conditioning and Dewatering

SLUDGE CONDITIONING AND DEWATERING

• Done to improve dewatering characteristics of sludge.

Methods:

(i) Chemical Conditioning,

(ii) Heat Treatment.

(i) Chemical Conditioning

• Done for raw or digested sludges.

• Chemical like alum, polyelectrolytes, ferric or aluminium salts, lime, ferric chloride are added.

• These chemicals make the sludge particles agglomerate and become denser. Charge neutralisation occurs between particles and they form a rigid and porous lattice structure and permits escape of water.

• The choice of chemicals depends on pH, temperature and ash content of sludge.

• Optimum dosage of ferric chloride and alum is 1 kg/m3 of sludge.

• Digested sludge because of high alkalinity requires huge chemical demand and therefore the alkalinity has to be reduced to save on the chemical. This is achieved by elutriation.

• Elutriation is the physical washing of sludge with low alkalinity water which reduces the chemical demand.

• Elutriation is done before chemical conditioning. The methods of elutriation

(a) Single-stage go ba

(b) Multi-stage

(c) Counter-current washing.

• The quantity of washing water required depends on the method adopted and hing water required depends on t alkalinity of sludge.

• Single stage washing requires 5 times more water than the counter-current and sbivog is therefore used only for small plants.

• The sludge and digestion water are mixed in a chamber using mechanical arrangements for a detention period of 20 seconds. The sludge is then settled in settling tanks and excess water is decanted.

• In counter-current washing, (single or multi-stage), the sludge and dilution water flow into the tank from opposite directions.

• The volume of wash water required is twice or thrice the volume of sludge elutriated.

• Physical unit operation used to reduce moisture content of the sludge and thus increase the solids concentration.

Methods:

(i) Air Drying in sludge drying beds.

(ii) Mechanical means - Vacuum filtration

- Centrifugation bab

- Pressure filtration

Purpose of Dewatering:

(i) Cost of transportation of sludge to ultimate disposal site is reduced (volume To reduction).

(ii) Ease in handling dewatered sludge.

(iii) Increase in calorific value of sludge. Incineration cost is less due to moisture reduction.

(iv) Rendering sludge odourless and non-putrescible.

(v) Reduce leachate production when disposed in landfills.

Sludge Drying Beds

• Suitable for locations where temperature is high (India).

• Sludge is applied on specially prepared open beds of land. (c)

• A sludge bed consists of bottom layer of gravel of uniform size and depth 30 cm over which is laid a bed of clean sand of depth 15 to 30 cm.

• Clean sand of effective size of 0.5 to 0.75 mm and uniformity co-efficient not greater than 4 is placed over the gravel.

• Open jointed pipes are provided as under drains in gravel layer to provide drainage to the liquid that passes through the sand and gravel layers. •

• Under drains are made of vitrified clay of atleast 10 cm o at a spacing of 6 m apart.

• Graded gravel is provided around the under drains in layers upto 30 cm with a minimum of 15 cm above the top of under drains.

• Drying beds adopted are 6 to 8 m wide and 30-45 m long.

• Minimum of two drying beds should be provided.

• The area needed for dewatering digested sludge is dependent on total volume of sludge, climate, temperature and location.

• Sludge should be deposited evenly to a depth of not greater than 20 cm.

• When digested sludge is deposited on a well drained bed of sand, the dissolved gases tend to buoy up and float the solids leaving a clear liquid at the bottom which drains through the sand rapidly.

• Major portion of liquid drains off in few hours and evaporation starts. The sludge cakes shrink.

• With good drying condition, the sludge will dewater and become fit for removal in 2-3 weeks with volume reduction of 20 to 40%.

• Dried sludge cake is removed by showel when moisture is less than 70%.

• Pick up trucks are used for hauling of sludge cakes.

Mechanical Methods:

• Vaccum Filtration (common)

• Filter Press

• Centrifugation

• Chemical conditioning is normally done prior to mechanical dewatering.

• Used to dewater raw or digested sludges.

Vacuum Filters:

• Vaccum filter consists of cylindrical drum over which is laid a filter medium of wool, cloth, synthetic fibre, plastic, stainless steel (mesh) (or) coil spring.

• The drum is suspended horizontally so that one half is submerged in sludge tank.

• Valves and pipes are arranged to produce vacuum on the inner side of filter medium.

• When the drum enters the sludge tank, the conditioned sludge is spread in thin layer over the filter media.

• As the drum rotates, the vacuum holds the sludge against the drum.

• This pulls out water from the sludge leaving a moist cake mat at the outer surface.

• The sludge cake on the filter media is scraped out before the drum enters again in the sludge tank.

• The rate of filtration is 10 to 50 kg/m²/hr.