Advanced Sludge Dewatering: What Filter Presses Can Reach, and Why Electro-Osmosis Only Has Test Values

1. Definition First, and the Gap against Reality

The Technical Guideline for Sludge Treatment and Disposal of Municipal Wastewater Treatment Plants (trial), issued under document Jian Ke [2011] No. 34, defines advanced dewatering as follows: advanced dewatering means the moisture content of sludge after dewatering reaches 55 percent to 65 percent, and under special conditions can be even lower. The same document describes the domestic situation thus: most municipal wastewater treatment plants in China have no primary settling tank and do not use anaerobic digestion, so the moisture content of dewatered sludge is mostly between 78 percent and 85 percent. The gap between these two figures is the entire market space for advanced dewatering.

Understanding that gap requires the whole moisture-content chain (Chapter 2 of the guideline):

• primary sludge typically 97 to 98 percent; waste activated sludge typically 99.2 to 99.8 percent;
• after thickening typically 94 to 96 percent; after dewatering it can be reduced to about 80 percent;
• mechanical thickening generally reduces waste activated sludge from 99.2 to 99.5 percent down to 94 to 96 percent.

The indicator that decides whether the sludge can be pressed at all is specific resistance to filtration, defined as the resistance per unit filtration area per unit mass of dry solids filtered, in m/kg. The guideline gives typical values of 20 to 60 x 10^12 m/kg for primary sludge, 100 to 300 x 10^12 m/kg for waste activated sludge, and 40 to 80 x 10^12 m/kg for anaerobically digested sludge, and states plainly that sludge above 1 x 10^13 m/kg is difficult to dewater, so conditioning must be applied before mechanical dewatering to reduce specific resistance.

2. The Performance Ceiling of Conventional Mechanical Dewatering

Section 3.2 of the guideline gives feed requirements and cake moisture content for four mechanical dewatering types.

• Belt filter press: low noise and low power, but large footprint and washwater demand and poor workshop environment; feed moisture generally below 97.5 percent, cake generally below 82 percent.
• Centrifuge: small footprint, no washwater, good workshop environment, but high power and polymer demand and high noise; feed 95 to 99.5 percent, cake generally 75 to 80 percent.
• Plate and frame filter press: low cake moisture, but large footprint and washwater demand and poor workshop environment; feed moisture generally below 97 percent, cake generally 65 to 75 percent.
• Screw press and roller press: small footprint, low washwater, low noise, good workshop environment, but small unit capacity and high solids in centrate; feed 95 to 99.5 percent, cake generally 75 to 80 percent.

The conclusion is direct: changing equipment alone cannot reach 55 to 65 percent. A plate and frame press dosed only with organic polymer tops out at 65 to 75 percent, exactly outside the advanced dewatering threshold.

3. The Real Variable Is Conditioning: the Full Values in Table 4-1

The guideline divides conditioning into chemical, physical and thermal types. Chemical conditioning uses inorganic metal salts, organic polymers and various sludge modifiers. Physical conditioning adds inert materials that do not react chemically - flue gas ash, diatomite, incinerated sludge ash and fly ash - to reduce or improve compressibility. Thermal conditioning includes freezing, mesophilic and thermophilic heating; high-temperature thermal conditioning is the common form and is subdivided into thermal hydrolysis and wet oxidation, which also achieve a degree of volume reduction alongside deep dewatering.

The mechanism is described as modifying the organic matter on the surface of sludge particles, or destroying the cell and colloidal structure of sludge to reduce its water-binding capacity, while simultaneously reducing compressibility so the sludge can withstand a high-dryness dewatering process.

Table 4-1 of the guideline gives cake moisture content (percent) for combinations of conditioning method and dewatering machine.

• Organic polymer dosing: belt filter press or centrifuge 70 to 82; plate and frame filter press 65 to 75.
• Inorganic metal salt dosing: plate and frame filter press 65 to 75.
• Inorganic metal salt plus lime: plate and frame filter press 55 to 65.
• High-temperature thermal conditioning: belt filter press or centrifuge 50 to 65; plate and frame filter press below 50.
• Combined chemical and physical conditioning: belt filter press or centrifuge 50 to 65; plate and frame filter press below 50.

In engineering terms: a conventional plate and frame press settles at 65 to 75; to cross into 55 to 65, metal salt plus lime is the lowest-threshold route; to go below 50 one must adopt high-temperature thermal conditioning or combined conditioning, at the cost of higher investment, energy use and filtrate load.

4. Electro-Osmotic Dewatering: Test Values Only, No Code Values

This must be stated plainly: neither the technical guideline above nor the current national standard system provides engineering design parameters for electro-osmotic dewatering. The quantified results available come from two published academic studies and are experimental values, not code values.

4.1 Optimised operating parameters from orthogonal experiments

A study published in Acta Scientiarum Naturalium Universitatis Sunyatseni, 2020, volume 59, number 6, ran two rounds of orthogonal experiments using cake moisture content and dewatering energy as the evaluation system and obtained the following optimised parameter combination:

• mechanical pressure 14.6 to 15.0 kPa;
• electro-osmosis time 58.7 to 70.0 min;
• voltage gradient 15.1 to 16.1 V/cm;
• sludge thickness 0.50 cm;
• cake moisture content 55.00 to 58.00 percent;
• energy consumption 0.110 to 0.120 kW.h/kg, equal to only 9 to 19 percent of the energy use of thermal drying.

4.2 Field application trials

An applied study on sludge from municipal wastewater treatment plants in Chongqing, covering field trials at four plants, reported:

• preferred operating conditions of voltage gradient 24 to 36 V, cycle time 10 to 20 min, feed thickness 8 to 12 mm, mechanical pressure 0.3 to 0.4 MPa;
• energy consumption for water removed when reducing moisture from about 80 percent to 50 or 60 percent of 0.190 to 0.250 kW.h/kg; of which 0.080 to 0.120 kW.h/kg to reach 60 percent and 0.120 to 0.130 kW.h/kg to reach 50 percent, per kg of raw sludge;
• low voltage with long time is preferred to minimise energy, high voltage with short time to maximise throughput;
• mechanical dewatering should dose polyacrylamide at 2 to 3 per mille and should not dose polyaluminium chloride or similar coagulants; electro-osmotic dewatering should follow mechanical dewatering as soon as possible.

The order-of-magnitude difference between the two data sets is itself the lesson: 15 to 16 V/cm and 24 to 36 V correspond to completely different cake thicknesses (0.50 cm versus 8 to 12 mm) and contact modes. Electro-osmotic parameters must be established by bench testing on the actual sludge; they cannot be transferred between projects.

5. Policy Constraints: the Landfill Route Is Closing

Much of the practical driver for advanced dewatering comes from tightening landfill rules.

• Implementation Plan for Harmless Treatment and Resource Utilisation of Sludge (28 September 2022), article (5): in regions not yet able to use land application, incineration or building-material utilisation, sanitary landfill may be used provided the sludge moisture content is below 60 percent; and sludge that has not been dewatered to standard is prohibited from landfilling at municipal waste landfills.
• Implementation Plan for Strengthening Weak Links in Municipal Wastewater Treatment Facilities (2020): restrict landfilling of sludge that has not been dewatered to standard, and accelerate reduction of sludge landfilling in prefecture-level and above cities in the eastern region and large and medium cities in the central and western regions.
• Technical Policy on Sludge Treatment, Disposal and Pollution Prevention and Control (trial), Jian Cheng [2009] No. 23, clause 3.4: sludge must be stabilised before landfilling, with shear strength greater than 25 kN/m2; the landfill shall have a gas collection and utilisation system and leachate meeting discharge standards.
• The 2011 technical guideline states: strictly limit and progressively prohibit direct landfilling of sludge that has not undergone advanced dewatering.

Comparing the "below 60 percent" requirement with Table 4-1 yields a practical criterion: to keep cake reliably below 60 percent, metal salt plus lime (55 to 65) sits in the marginal zone, while high-temperature thermal or combined conditioning (below 50 on a plate and frame press) provides margin.

The same 2022 plan sets the scale targets: by 2025, the newly added nationwide capacity for harmless sludge treatment and disposal shall be not less than 20,000 tonnes per day (wet sludge at 80 percent moisture), with the harmless disposal rate reaching above 90 percent in cities and above 95 percent in prefecture-level and above cities.

6. Secondary Pollution and Acceptance Criteria

• Odour: the main sources in thickening and dewatering are the sludge storage tank, thickener, dewatering room and sludge stacking area or silo; odour in the dewatering room does not disperse easily and is the focus of odour control. New plants shall enclose the thickener, storage tank, dewatering room and sludge handling area, extract air with make-up air and send it to an odour treatment system; biological deodorisation is generally used, with chemical deodorisation where necessary.
• Filtrate: dewatering filtrate is normally returned to the plant inlet; where ammonia nitrogen or phosphorus is high, separate treatment should be considered. Sludge from plants using biological phosphorus removal, and sludge held too long under anoxic conditions, releases large amounts of phosphorus and directly raises filtrate total phosphorus.
• Acceptance criteria: design documents should state together the target moisture content, the conditioning chemicals and their dosages (to be fixed by testing), the assessment of filtrate return shock on the biological treatment unit, the odour control method and emission standard, and the sludge quality standard corresponding to the intended cake destination.

Sources: Section 2.1 of Chapter 2, Section 3.2 and Table 4-1 of Chapter 4 Section 1, and Chapter 4 Section 4 of the Technical Guideline for Sludge Treatment and Disposal of Municipal Wastewater Treatment Plants (trial), Jian Ke [2011] No. 34; Implementation Plan for Harmless Treatment and Resource Utilisation of Sludge (28 September 2022); Implementation Plan for Strengthening Weak Links in Municipal Wastewater Treatment Facilities (2020); Technical Policy on Sludge Treatment, Disposal and Pollution Prevention and Control (trial), Jian Cheng [2009] No. 23. Electro-osmosis parameters are experimental values quoted from Acta Scientiarum Naturalium Universitatis Sunyatseni (2020, vol. 59, no. 6) and an applied study on electro-osmotic dewatering of sludge from municipal wastewater treatment plants in Chongqing.