Filter Presses for Sludge Treatment1

Filter press performance depends on type of Sludge which is going to be filtered. To obtain higher efficiency & better performance in Filter Presses to treat all kind of Sludge some conditioning agents are added on filter paper or filter cloth. So, there is a lot of research & development going on around the globe for developing best suited & economical Conditioning Agent or Flocculants. Diatomaceous Earth (DE), Fly Ash are most commonly used Bulking Agents and Ferric Chloride, Alum, Lime, Polyelectrolyte(polymer) are most commonly used Conditioning Agents.

Types of Sludge are:-
  • Hydrophilic organic sludge: Inorganic conditioning is often recommended to enable satisfactory cake release due to minimal adherence to filter cloth.
  • Hydrophilic inorganic sludge: The filter press generally requires the addition of lime only.
  • Hydrophobic inorganic sludge: It is very dense and ideal for the filter press. It is dewatered without any preliminary conditioning.
  • Oily sludge: The filter press can be used to treat sludge containing light oils, the presence of grease can sometimes impair the smooth running of the filter; clothes have to be degreased at frequent intervals.

The filter cloths are woven fabrics using monofilament or multifilament synthetic fibers or a combination of both. The most commonly used materials are polypropylene, polyester, and nylon.


Candle Filter Technology2

Candle Filters are installed for clarification and recovery applications from liquids with low solids content from 5% to less than 1% solids and even down to trace amounts.

Automatic Process Cycle:-

Filling: The slurry feed enters the bottom of the filter vessel.

Filtration: The slurry is pumped under pressure into the vessel. Cake will deposit on the outside of the candle; the separated filtrate will flow through the filtrate pipe and the registers. This process continues to a maximum pressure drop, the maximum cake thickness, or the minimum flow.

Washing: Displacement washing or recirculation washing.

Drying: Blowing gas, steam or “shock” drying.

Heel Filtration: The liquid remaining in the vessel cone after filtration or washing is completely filtered.

Cake Discharge: Gas flows through the register pipes, and down the filtrate pipe. The filter media gently expands allowing for cake discharge. Alternatively, the cake can be discharged as a slurry.

Sock Cleaning after Cake Discharge: If necessary, sock cleaning is accomplished by filling and circulating cleaning fluids while blowing gas in the reverse direction to the filtration direction, which creates a turbulent mixture or a quasi-ultrasonic cleaning effect.


Energy efficient Membrane Filter Press 3

From energy use and technology gap audit studies in various chemical industries in Ahmedabad chemical cluster, below mentioned things are identified:

  • Energy efficiency improvement opportunities
  • Environment and safety improvement of workers
  • Design flaws in the conventional filter press
  • Operational & maintenance practices in conventional filter press

The drying cost is one of the major costs in the overall production process of chemicals, in typical chemical industry. Apart from the energy cost, drying time is one of the major time consuming area in overall production process of chemicals, in typical chemical industry the drying time would be around 72 hours per batch. The existing installed conventional dryer takes 72 hours for processing one batch. If combined with a membrane filter press the drying time is reduced to 49 hours per batch. This not only saves energy consumed by the dryer, through reduced hot air consumption for hot air generator but also enhances the productivity of the dryer as the drying process becomes faster process.

Advantages of replacing the conventional filter press with Energy Efficient membrane filter press are:
  • Reduction in load on HAG and hence wood consumption
  • Improved productivity
  • Improved working environment
  • Faster filter press process, it leads to energy savings
  • Improves the efficiency of the unit
  • Reduction of deforestation and GHGs emissions
Sr. No. Details Conventional Filter Press Energy Efficient Membrane Filter Press
1 Drying time in Dryer due to caking More Less
2 Load on Hot Air Generator (HAG) More Less
3 Environment pollution High (partial combustion & more fuel consumption) Low ( Complete combustion & less fuel consumption)
4 Caking Time High Low
5 Operational Cost ( due to compressed air usage) High Low
6 Heat Losses High Low

From the above table it is clear that Energy efficient membrane filter press has significant advantages in Energy, Environmental, Economic & safety aspects over the conventional filter press. It is therefore, justifiable to install energy efficient membrane filter press in place of conventional filter press. – Detailed Project Report (DPR) on Membrane Filter Press, DPR No. AMD/CHM/MFP/07

Detailed Technology Assessment Report (from Detailed Energy Audit Report on Membrane Filter Press done by BEE at Chemical SME Cluster, Ahmedabad, Gujarat, India.)
Sr. No. Parameter Units Value
1 Drying time in conventional filter press hrs/batch 72.00
2 Drying time in new membrane filter press hrs/batch 49.00
3 Volume processed per batch litre 1280.00
4 Wood consumption in HAG (Hot Air Generator) Kg/hr 70.00
5 Wood consumption in HAG with conventional filter press Kg/batch 5040.00
6 Wood consumption in HAG with membrane filter press Kg/batch 3430.00
7 Specific fuel consumption for conventional filter press (@ 1280 Litre/batch ) Kg/litre 3.94
8 Specific fuel consumption for membrane filter press (@ 1280 Litre/batch ) Kg/litre 2.68
9 Savings in specific fuel consumption Kg/litre 1.26
10 Annual operating batches Nos 150.00
11 Annual production capacity @ 1280 Litre/Batch Litre/annum 192000.00
12 Wood energy saved per annum tonnes/annum 241.50
13 Average fuel cost Rupee/tonne 3000.00
14 Total monetary saving rupee( in Lakh) 7.26
15 Total investment required Rupee( in Lakh) 24.20
16 Simple payback period Years 3.33