Countercurrent washing in wash boxes (compartments)
Technical description
In woven fabric textile finishing industries, the fabric is washed in a number of wash boxes (compartments) in continuous machines. Generally, the wash boxes of the machine vary from two to eight. In the following, processes are mentioned which are carried out in continuous machines where fabric is washed in wash boxes:
- Pretreatment (washing for desizing, scouring, bleaching)
- Mercerization
- Washing after pad steam dyeing
- Soaper machine (soaping and washing after printing)
- Washing machines for washing fabric after cold pad batch dyeing or fabric printed with reactive or vat dyestuffs
The latest machines are designed in counter-current washing mode in which all the wash boxes are interconnected, and the washing water of one wash box is used in the other wash box (see Figure 1). In the counter-current washing mode, water and energy consumption is reduced substantially. The locally manufactured machines are often not designed in counter-current washing mode where each wash box is operated independently with the supply of water in the wash box which is heated through steam and then wasted as overflow water. There is a substantial amount of water and energy consumption in such a type of arrangement. Counter-current washing is based upon the principle that the dirty fabric is in contact with dirty water and the clean fabric is in contact with clean water because the fabric and water move counter-currently.
In some cases, even though the machine is operated in the counter-current mode, operators disconnect this arrangement and do not operate the machine in the counter-current mode due to their ease of operation.
It is a resource-efficient way to operate machines in counter-current washing mode to save water and energy. In the counter-current washing mode, the hot washing water of one box is transferred to another wash box, and so on. In this way, the water and energy of one wash box are utilized in the adjoining wash box.
Achieved environmental benefits:
- Water conservation
- Reduction in thermal energy (hot water will require less steam for heating)
- Reduction in greenhouse gases
- Reduction in a hydraulic load of the wastewater treatment plant
Operational data:
The quantity of water and energy reduction will vary from industry to industry depending upon the number of wash boxes, flow rate of water supply, temperature of washing water and operational frequency of the machine. In Table 1 below, specific water consumption figures for different continuous washing processes are compiled which can be achieved in practice by applying counter-current washing, efficient washing boxes, and machine-specific daily monitoring of water consumption.
Table 1: Typical achievable figures for the specific water consumption of various continuous washing operations for woven fabric (with one exemption which is continuous demineralization of cotton knitwear)
Source: Source: Schönberger, H.; Schäfer, T., Best Available Techniques in Textile Industry, UBA-Texte 14/03, https://www.umweltbundesamt.de/sites/default/files/medien/publikation/long/2274.pdf - all figures up-dated in November 2020 (however, only few changes were required compared to the figures from 2003)
For one typical industry where two pretreatment machines operate without counter-current washing mode, the water and energy saving in monetary terms will be USD 191,260/yr (Rs. 28.69 million/yr). Water conservation will be 590 m3/d (177,000 m3/yr).
The implementation cost will be USD 13,330 (Rs. 2 million) for the re-arrangement of the wash boxes and their connection with each other in the two machines.
Cross-media effects:
No relevant cross-media effect is known.
Technical considerations relevant to applicability:
In case the wash boxes of the machine are already placed in such a way that a gradient exists for water flow from one box to another, then the only thing will be to connect these boxes with each other. In cases where wash boxes of the machine are placed at the same floor level and there is no such gradient, the machine’s boxes will have to be relocated to create a gradient for flow.
Economics:
Capital cost = USD 13,330 (Rs. 2 million)
Annual saving = USD 191,260 (Rs. 28.69 million)
Annual O&M cost = Nil
Simple payback period = 1 month
Additional benefits will be the reduction in hydraulic load and treatment cost of the wastewater treatment plant.
Driving force for implementation
- Financial benefits
- Resource conservation
- Sustainability of textile business
Reference industry:
This option is relevant for all the woven fabric textile finishing industries where continuous washing of fabric is carried out in different machines as mentioned above.
Back