Reduction of COD in textile effluent

[nklprsd]

Hello,

I have been asked to look at treating the effluent from a textile unit, which does dyeing and bleaching process. The following average values have been measured for the effluents, which have been segregated based on their COD values.

Dyeing effluent:
Flow - 800 m3/hr
COD - 800 mg/l
BOD - unavailable
pH - 7 to 8

Bleaching effluent:
Flow - 400 m3/hr
COD - above 3000 mg/l
BOD - unavailable
pH - 10 to 12

The basic scheme is as follows:

(i) Pre treatment - Bar screen chamber (one for dyeing and one for bleaching stream)- Equalization Tank (one for bleaching and one for dyeing). The overflow from the equalization tank for bleaching effluent enters the equalization tank for dyeing effluent. The combined effluent then goes for further treatment

(ii) Primary treatment - Coagulation & Flocculation followed by Inclined Plate Settler

(iii) Secondary treatment - Sequencing Batch Reactor

(iv) Tertiary Treatment - Coagulation & Flocculation followed by Inclined Plate Settler

Recently, the flow for the dyeing stream has reduced drastically and has come closer to the flow rate of the bleaching stream. Infact, the bleaching stream flow rate is slightly higher. As a result,the COD of the effluent is not reducing much. The COD from the outlet of the SBR comes to only around 400 mg/l. Any suggestions for what can be done to reduce the COD further? Ideally a value of below 150 mg/l is what is required.

 

[bimr]

If the wastewater contaminants are biodegradable, you may add an additional activated sludge process in series.

 

[nklprsd]

The contaminants are not so biodegradable and hence, the COD levels are not coming down. I am not sure if an additional activated sludge process is required when a biological treatment process is already in place.

 

[moltenmetal]

If you've removed all the solids and are left with dissolved, non-biodegradable COD, your only option for further reductions are either a) source elimination or b) advanced oxidation. Fenton's reagent (lowering the pH to 3 followed by an addition of ferrous sulphate and hydrogen peroxide) or direct ozononation are easy options, but their effectiveness will depend greatly on the composition of the water.

 

[bimr]

The efficiency of the activated sludge process is limited to 80-90% removal. By adding an additional stage, it is possible for further reduction of biodegradable material. The SBR process would not be the preferred process.

If the material is not biodegradable, other steps as suggested by Moltenmetal should be investigated.