Coagulation and Flocculation: Chemicals, Dosing Systems and Hydraulic Parameters in HJ 2006-2010

1. Be clear about what coagulation removes and what it does not

HJ 2006-2010, Technical specifications for coagulation and flocculation process in wastewater treatment, was approved by the former Ministry of Environmental Protection on 2010-12-17 and took effect on 2011-03-01. It applies to the design, construction, acceptance and operation management of coagulation and flocculation processes in municipal or industrial wastewater treatment projects. The boundary statements in chapter 4 are worth quoting: clause 4.2 states that the process imposes no limit on the concentration of suspended solids, colloidal particles, associated organic matter, colour causing substances and oil in the raw water, while clause 4.3 says it has a good removal effect on suspended particles, colloidal particles and hydrophobic pollutants, and a considerable flocculation effect on hydrophilic and dissolved pollutants. Read together, coagulation acts on particulate and colloidal pollutants, and compliance for dissolved COD must not be entrusted to coagulation.

Clause 5.3 fixes the overall reaction pH: the pH at which the coagulation reaction is completed varies greatly with the chemical and dose, and the optimum pH is 7 to 8.5. Clause 5.2 requires pretreatment such as equalisation and oil separation upstream, and a sedimentation tank or flotation tank downstream, and clause 5.7 stresses that solid liquid separation is accomplished by the downstream sedimentation or flotation equipment, so the sludge produced must be brought into the unified sludge treatment and disposal plan - the point most often challenged in environmental impact assessment and completion acceptance. Clause 6.1.1 gives the combined construction criterion: where the treated flow is below 100 m3 per hour, the coagulation process should be built integrally with the sedimentation or flotation tank, and clause 6.1.6 requires sludge withdrawal facilities in the reactor where suspended solids or chemical doses are high.

2. Chemical selection: coagulants, flocculants and coagulant aids each have a role

2.1 Coagulants (Table 1 and clause 6.2.1)

• Aluminum sulfate: suitable for raw water of high pH and high alkalinity; for emulsion breaking and removal of organic matter the preferred pH is 4 to 7, for suspended solids removal 6.5 to 8, with an applicable water temperature of 20 to 40 degrees C;
• Ferric chloride: corrosive to metals, concrete and plastics; applicable pH range preferably 7 to 8.5, with rapid formation of stable floc and short settling time; ferrous iron must first be oxidised to ferric iron, requiring aeration or chlorine dosing at low pH; quality shall comply with GB 4482;
• Ferrous sulfate: the raw water must have sufficient alkalinity and dissolved oxygen, with pH generally controlled above 8 to 8.5; the chlorine dose follows equation (2), generally one eighth of the FeSO4.7H2O dose with an excess of 1.5 to 2 mg/L;
• Polyaluminium chloride (PAC): little affected by pH and temperature with stable adsorption; a wide applicable pH range of 6 to 9 and generally no need to add alkali; quality shall comply with GB 15892, and most importantly the basicity B shall be between 50 and 80 percent, verified by equation (1);
• Polyferric sulfate (PFS): also a polymeric salt, particularly effective for high turbidity raw water.

Clause 6.2.1.4 adds a criterion that is frequently overlooked: where the wastewater contains heavy metal ions, iron salt coagulants shall be preferred. At the same time iron salts must not be overdosed and reaction conditions such as pH must be controlled, and the heavy metal and other pollutant content of the chemical must be verified before use; chemicals exceeding the limits shall not be used.

2.2 Flocculants and coagulant aids (clause 6.2.2)

PAM is the most common flocculant, and the text imposes four hard constraints: PAM is applied for flocculation after the aluminum or iron salt coagulation reaction is complete, at a dose generally below 0.3 to 0.5 mg/L, dosed at the end of the reactor; a dedicated dissolving and hydrolysis unit is required, with a dissolution time of 45 to 60 minutes, a prepared concentration below 2 percent, a hydrolysis time of 12 to 24 hours and a degree of hydrolysis of 30 to 40 percent; the solution must not be used after more than 48 hours; and antifreeze measures are needed for ambient storage. Coagulant aids may be chlorine, lime or sodium hydroxide: chlorine for high colour water and to promote oxidation of ferrous iron; lime to supplement alkalinity, remove carbon dioxide, adjust pH, increase floc density and improve sludge dewaterability; sodium hydroxide simply to adjust pH.

3. Dosing system: quantified parameters from preparation to addition

Chapter 6.3 gives a complete quantified set: water supply pressure for hydraulic preparation shall exceed 0.2 MPa; compressed air preparation is controlled at an aeration intensity of 3 to 5 L per square metre per second, and is unsuitable for lime emulsion; the dissolving tank volume W1 equals 0.2 to 0.3 times W2, with the solution tank volume calculated from equation (4); solution concentration is generally 5 to 20 percent by weight of solid coagulant, or 5 to 7.5 percent after deducting water of crystallisation, and 2 to 5 percent for lime milk calculated as pure CaO; and the number of preparation batches per day should generally not exceed three. Clause 6.3.2.3 requires the floor slope of dissolving and solution tanks to be not less than 0.02, with a sludge discharge pipe, freeboard and internal corrosion protection, a standby solution tank, and a stirring speed generally of 10 to 15 revolutions per minute. Clause 6.3.3.2 requires metering pumps with automatic control for dosing. Clause 6.3.4 is equally specific about the chemical store: the fixed reserve may be taken as 7 to 15 days of maximum dose, the stacking height of coagulant is generally 1.5 to 2.0 m, or 1.5 m for lime, and the working platform beside the solution tank is preferably 1.5 m wide.

4. Mixing and flocculation: G value, GT value and the three stage folded plate

4.1 Mixing (clause 6.4)

On selection: an in line static mixer suits stable flow, water free of fibrous material and a pump with spare head; pump mixing suits low silt and low suspended solids with the pump close to the reactor; mechanical mixing suits complex raw water with variable flow and quality and the mixing basin may be combined with the flocculation basin. Operating parameters are: a mixing time generally of 10 to 30 seconds with a velocity gradient G of 600 to 1000 per second; a connecting pipe velocity of 0.8 to 1.0 m/s between the mixing device and the downstream structure; and one prohibition - flocculation aids such as polymeric flocculants that increase floc formation shall not be dosed in the mixing device.

4.2 Flocculation (clause 6.5)

The overall parameters are: a reaction time generally controlled at 15 to 30 minutes; an average velocity gradient G of 70 down to 20 per second with a GT value of 10 to the power 4 to 10 to the power 5, both G and the flow velocity decreasing progressively; the reactor should be built integrally with the sedimentation or flotation tank, and where a pipe connection is unavoidable its velocity shall be below 0.15 m/s; the velocity through the outlet perforated wall is preferably below 0.10 m/s. The vertical flow folded plate reactor has three stages (clause 6.5.3.1): first stage with opposite wave plates at G equal to 80 per second, T of at least 240 seconds and v of 0.25 to 0.35 m/s; second stage with parallel wave plates at G equal to 50, T of at least 240 seconds and v of 0.15 to 0.25 m/s; third stage with straight parallel plates at G equal to 25, T of at least 240 seconds and v of 0.10 to 0.15 m/s. The fold angle may be 90 to 120 degrees and the plate length 0.8 to 1.5 m. Head loss formulae and resistance coefficients are also given for each stage - gradual expansion coefficient 0.5, gradual contraction 0.1, upper turn 1.8 and lower turn or orifice 3.0 - which can be used for hydraulic profile checks.

Two reading points deserve attention. First, clause 6.5.2 writes the G value as "70 to 20 per second", which is how the released draft expresses it; combined with the requirement that it decrease, it should be understood as 70 per second at the inlet falling to 20 per second at the outlet. Second, the grid or bar reactor is explicitly unsuitable for wastewater with much fibrous or oily material, whereas the mechanical reactor suits all industrial wastewaters and allows adjustment to changing flow and quality - these two are hard constraints when selecting a reactor type for an industrial project.

5. The list of parameters that must be determined by testing

The specification repeatedly puts testing first. Clause 6.1.2 requires that the type and quantity of chemicals be determined by testing according to the raw water quality (pH, alkalinity, suspended solids) and the nature of the pollutants (relative molecular mass, molecular structure, density, concentration, hydrophobicity); clause 6.2.1.2 requires the coagulant type and dose to be determined by a coagulation sedimentation test on the wastewater or by comparison with operating experience under similar water quality; and clause 6.3.1 restates that chemicals and their doses shall be determined by testing on the wastewater characteristics. In other words, Table 1 provides applicability conditions, not doses: copying the dose of a comparable project without running a coagulation sedimentation test does not comply with this specification.

In addition, the specification does not give surface loadings or retention times for sedimentation or flotation tanks (clause 5.2 only requires that such units be provided downstream, and clause 6.3.2.3 refers to the dedicated flotation specification), nor any relationship between chemical dose and effluent total phosphorus for chemical phosphorus removal. Dosing for chemical phosphorus removal must be determined by testing against the influent phosphate concentration and the target effluent value, and this specification cannot serve as the basis for the dose.