Biological treatment of textile wastewater, mainly carried out in activated sludge systems

 

Technical description

 

The objective of biological wastewater treatment is to i) biodegrade water-soluble organic biodegradable compounds and insoluble organic matter, ii) transform (i.e. partly oxidize) dissolved and particulate biodegradable constituents into non-hazardous metabolites, iii) capture and incorporate suspended and non-settleable colloidal solids into biomass, and iv) to adsorb non-biodegradable compounds to biomass (activated sludge flocs and biofilm).

The removal of dissolved and suspended organic matter is accomplished biologically by a huge variety of microorganisms, mainly bacteria. Microorganisms are used to biochemically oxidize (i.e. to transform) the dissolved and particulate organic matter into mainly carbon dioxide, water, ammonium, sulphate, phosphate, some, often unknown, biochemical transformation products (metabolites) and additional biomass (sludge) which is the results of microbial growth of (mainly) bacteria.

Technically, biological degradation processes used for wastewater treatment can be divided into two main categories: suspended growth and attached growth (or biofilm) processes. Activated Sludge Wastewater Treatment Plants (ASWWTP) still dominate by far. As a consequence, ASWWTPs are described in more detail here both, conventional and advanced systems such as membrane bioreactors. It is based upon the suspended growth where the concentration of the microorganisms, responsible for treatment, is maintained in liquid suspension by appropriate mixing methods. Today, this is usually performed by fine bubble aeration from the bottom whereas mechanical surface aerators are not considered appropriate anymore. ASWWTPs are aerobic processes. Therefore, bacteria have to be supplied with dissolved oxygen. However, ASWWTP can be combined with anaerobic stages before activated sludge. 

Components: The following are the major components of the ASWWTP:

  • Screening chamber
  • Equalization tank
  • Neutralization chamber
  • Primary clarifier (sedimentation tank) – only required in case the wastewater contains settable suspended solids or floated material, such as material from stone washing, or if precipitation is applied (so-called physical-chemical treatment - usually with iron or aluminum salts and a polyelectrolyte).
  • Aeration tank with fine bubble aeration from the bottom
  • Secondary clarifier (sedimentation tank); in some cases, a membrane biological reactor (MBR) is applied; then, the clarifier is substituted by ultrafiltration membranes which are arranged externally or are submerged in the aeration tank
  • Sludge dewatering and drying (chamber filter press, belt press, drying beds)

 

Figure 1 shows a conventional typical set-up of an activated sludge system with primary and secondary clarifiers.

 

Figure 1: Process scheme of a conventional activated sludge textile wastewater treatment plant

Figure 2 shows the scheme of a biological textile wastewater treatment plant with a membrane bioreactor and a 3-stage reverse osmosis plant to recycle wastewater. Here, the membrane bioreactor has an external ultrafiltration membrane that separates wastewater from microorganisms (biomass). It is also possible to submerge the ultrafiltration membrane in the activated sludge tank.

 

Figure 2: Process scheme of a biological textile wastewater treatment plant with a membrane bioreactor with external ultrafiltration membranes so separate the biomass from water; this plant is also equipped with a 3-stage reverse osmosis unit for water recycling (Schoenberger, 2018)

Achieved environmental benefits:

The organic biodegradable, as well as non-biodegradable but adsorbable pollutants of textile wastewater are removed through ASWWTP. So, the depletion of dissolved oxygen (DO) and the adverse impact of harmful but biodegradable or adsorbable organic compounds in the river water and contamination of river beds due to the removal of biodegradable organic compounds and suspended solids can be solved. The improved DO level, the less deposition of suspended solids at the river bed, and the removal of harmful organic compounds result in survival and better conditions for the aquatic life of the rivers (aquatic organisms and plants).

Operational data:

As an example, typical wastewater characteristics of a textile finishing industry are given below for which ASWWTP is installed.

Table 1: Example of textile wastewater flow and characteristics 

Type: wastewater from finishing woven fabric (pretreatment, dyeing, printing, and final finishing)

Daily wastewater flow (m3/day)

2,500

pH

9

BOD5 (mg/l)

600

COD (mg/l)

1,200

TSS (mg/l)

500

Oil and grease - O&G (mg/l)

30

The basic design of an activated sludge plant follows the food-to-microorganism ratio (F/M). To have an advanced removal efficiency, the F/M should be lower than 0.3 kg BOD5/kg MLSS x d for a wastewater temperature of higher than 30°C in the activated sludge tank which is usually the case. There are plants with extended aeration which means that F/M is lower than 0.1 kg BOD5/kg MLSS x d. However, then the microorganisms are in maintenance mode and their enzyme systems do not run at full capacity and excess sludge formation is very low. Further, the plug flow mode is the most efficient one, more efficient than completely stirred mixed reactors.

Cross-media effects:

There will be solid waste generation in the form of excess sludge (see Figure 1) from activated sludge and, if present, from the primary clarifier. The wastewater can become septic and odorous in case of malfunctioning of the mixing and aeration system of the treatment plant. 

Technical considerations relevant to applicability:

Activated sludge is a broadly applied technique for biological textile wastewater treatment. The wastewater from wet processes of woven, knitted and denim fabric and from yarn dyeing such as desizing, scouring, bleaching, mercerization, dyeing, printing, and finishing can be treated with this technique.

The following operational parameters need to be maintained in the aeration tank for the optimum operation of the system:

pH                                                          7 - 9

Temperature                                          max. 37°C

Dissolved oxygen                                  1.5 – 2 mg O2/L

Nutrients required ratio (BOD5:N:P)     100:5:1 (weight basis)

Sludge Volume Index (SVI)                   100 - 150

(Settled sludge volume (ml/l) in 30 min/MLSS (mg/l)

The plant operator will require training for operating the wastewater treatment plant from the technology vendor or consultant initially. The training will include an understanding of all the components of the plant concerning its operation and process control, occupational health and safety aspects, and troubleshooting.

There should be an onsite laboratory to frequently test the wastewater parameters to ensure its optimum operation all the time. In case of any fluctuation in the influent quality, reasons should be investigated and corrected immediately to avoid malfunctioning of the plant.

The required land area for the installation of the plant will be 0.3 m2 per m3/d wastewater i.e. 750 m2 area for 2,500 m3/d wastewater treatment.

Economics:

Conventional plant

Capital cost = USD 450 per m3/d wastewater i.e. USD 1.1 million for 2,500 m3/d wastewater treatment.

Operating cost = USD 0.3 per m3 i.e. USD 274,000 per year

Modern plant with membrane bioreactor

Capital cost for a capacity of 3500 m3/d (civil construction and machinery, including 3-stage reverse osmosis plant) = 4 million USD

Operating cost (except reverse osmosis stage): 0.67 USD/m3

Operating costs for reverse osmosis stage: 0.21 USD/m3

Driving force for implementation:

The driving force for the installation of ASWWTP is to ensure the treatment of wastewater and achieve the desired quality of the effluent that complies with legal requirements and discharged effluent in the river system does not affect the river’s environmental quality.

In areas with water scarcity, water recycling is required. Then, ASWWTP performance must be best to remove all biodegradable compounds. In that and also in case of space constraints, membrane bioreactors are more efficient.

Reference industry:

There are many textile finishing industries in Pakistan and other countries where ASWWTP is installed and operating efficiently. Membrane bioreactors are increasingly applied.

References:

Schönberger, H., Technique combinations to meet the ambitious ZDHC Wastewater Guidelines, Proceedings of the Colloquium on Textile Wastewater Management 2018-09-18/Integrated Best Available Wastewater Management in the Textile Industry, Stuttgarter Berichte zur Siedlungswasserwirtschaft, Band 241, ISBN 978-3-8356-7411-0, Vulkan-Verlag GmbH, Essen (2018) 35 - 70


Back