MBR Membrane for wastewater Chemical offline Cleaning Steps and Recovery cleans Process

Standard chemical cleaning is a key to managing chemical fouling & microbiological growth in the permeate pipes and to sustaining high membrane performance. Standard chemical cleaning should be started if the membrane is fouled (trans- membrane pressure has risen by 50 mbar from its initial operating level at the same permeated water flow rate), or if increased levels of Total Coliform or E.Coli bacteria are determined.

From a preventive maintenance point of view, more frequent standard chemical cleanings applied with lower chemical concentrations are more effective in sustaining high permeability and low bacteria counts in permeate than less frequent cleanings with higher chemical concentrations.

• Typical Design of Standard Chemical Cleaning

 The type of chemical cleaner and the frequency of cleaning will depend on the type   of foulant and the rate of fouling. Organic and biological fouling is inherent in MBR plants since they are biological in nature. Inorganic fouling may also be present depending on the wastewater characteristics and chemicals added to the treatment process (e.g., metal salts addition for phosphorus removal). Standard conditions are described in the chapter 3.5.3 and from a preventive point of view, recommended standard chemical cleaning is at lower concentrations and higher frequency:

Perform standard chemical cleaning with 1,000-2,000 ppm NaOCl. Perform standard chemical cleaning with 2 g/l citric acid at pH 2.

Automate the possible parts of cleaning procedure.

Perform standard chemical cleaning when the plant inlet flow is low (e.g. at night). Adjust the frequency of cleaning, the type of chemical cleaner, and concentrations based on actual membrane performance and microbiological permeate quality.

• Cleaning Solution Preparation and Cleaning Procedure 

The concentration and necessary quantity of each chemical are shown in Table 3.1.   If the delivered concentration is higher than shown below, dilute it in the feed tank with clean water.

Chemical	Solution concentration	Amount used
Sodium hypochlorite	2,000-6,000 mg/l (Effective chlorine concentration) (pH is about 12) Preventive: 1,000-2,000 mg/l	5 l/element (RMBR-1520) or 3 l/element (RMBR-1010)
Oxalic acid	5-10 g/l
Citric acid	10-30 g/l Preventive: 2-5 g/l

 

Table 3.1 Chemical preparation guideline  Cleaning procedure (see above Figure 3.1):

1. Measure permeability (flux rate, TMP and liquid temperature) before starting chemical
2. Check that the chemical injection valve is closed and that the chemical feed pump is at
3. Feed the feed tank with specified amount of chemical and dilute if
4. Stop filtration and air diffusion and close the permeated water
5. Start the chemical feed pump and check overflow from head
6. Slowly open the chemical injection valve to inject the chemicals. Adjust the      injection rate by controlling the valve to achieve recommended injection time (around 30 minutes). A fast injection may lead to insufficient fulfillment of chemical to the
7. After injecting the specified amount of chemicals, stop the chemical feed pump. If specified amount cannot be injected, check the injection rate and injection piping. NEVER force to inject surplus chemical which may result in severe damage to the
8. Leave the equipment for 1 to 3
9. Close the chemical injection valve, open the permeated water valve and restart filtration. Chemicals remain in the permeated water in an early phase of filtration (for a period of 2 or more intermittent cycles). If it is inconvenient for the plant operation, send the permeated water back to the raw water tank or dispose of it in accordance with applicable legal standards for waste