Technical Articles
Issue: March , 2012
Biological Treatment of Textile Effluents: Best adoptable option results in cost & environment savings with outstanding treatment efficiency

Md. Rashaduzzaman  Mithun                                                                        



Abstract:
Textile wet processes generate effluents of extremely variable composition and constitute an environmental risk specially water pollution of major concern.  By rapid disposal of untreated wastewater deteriorate the ecosystem and damage the aquatic life.  To combat ecological degradation it is mandatory for a textile dyeing industry to install an effective effluent treatment plant (ETP).   This study, focused on investigating the best possible solution to install the most economic and effective effluent treatment plant analyzing  physico-chemical, biological, chlorination  and other combined wastewater treatment techniques practiced in composite textile  industries at present times in Bangladesh.  It is found that biological wastewater treatment possesses high degree of efficiency with minimum running cost (5 to 6 times less) compared to other methods. Average reduction efficiency of BOD, COD, TSS and TDS is found 84%, 59.1%, 81.7% and 54.8% respectively by biological treatment. None of the methods except biological method can satisfy discharge standard.  Combined physico-chemical and biological method is considered as the most efficient method.  But real situation is different.  High running cost (20-28 tk/m3) due to high chemical consumption, huge amount of highly toxic sludge (2-5 kg/m3) disposal problem do not encourage the owners to run the plant effectively. Treatment with chlorine is very cheap but high probability of producing dangerous disinfection byproducts (DBPs) restricts its application.

Keywords: Effluent treatment plant (ETP), biological treatment, efficiency, cost, environment


1.  Introduction:
      
The textile processing industries produce wastewater containing suspended solids, large amount of dissolved solids, unreacted dyestuffs, recalcitrant organics, toxicants, inhibitory compounds, surfactants, chlorinated compounds (AOX- Adsorbable Organohalogens), and other chemicals that are used in different stages of dyeing, �?xing, washing and other processing.  Signi�?cant amount of dyes are lost in the wastewaters.  Reactive dyes are easily washed off during the dyeing process, therefore, the residue dye always presents in the dye bath effluent, as much as 50% of the initial dye load [1].  Khan et al. [2] reported that a semi-automated composite textile industry of 10 ton capacity produces 1250 m3of effluent each day, which contains an assortment of chemicals including salts, dyes and bleaches.  Under the 1997 Rules fabric dyeing and chemical processing industries are categorized as “Red industries”, which is the highest category in the Rules and for which an effluent treatment plant (ETP) is mandatory. 

At present times four  methods which are usually used in textile wastewater treatment in Bangladesh are: 1. physico-chemical methods, 2. biological activated sludge(ASP) methods, 3. combined  physico-chemical and biological methods and 4. oxidation methods with chlorine gas.  The physico-chemical methods with coagulation-flocculation  can treat effectively the color and BOD in the influent. However, the disadvantages of this method are the high chemical cost, large amount of highly toxic physico-chemical sludge and can not satisfy national effluent quality standard (NEQS) [7]. Biological treatment is the most economic and eco-friendly process due to least running cost, no hazardous chemicals are required and very low non-toxic sludge are produced.  In this method waste water is treated by microorganisms mainly bacteria.  The use of microorganisms to remove contaminants from wastewater is highly effective and widespread.  Combination of physico-chemical and biological method is the most efficient method but high initial investment, unwillingness to operate the plant correctly due to high running costs  and maintenance problems  do not encourage the owners to install this combined method [7].  It is seen that wastewater treatment with oxidants like Chlorine (Cl2) is now practicing in Bangladesh. Chlorine can react with natural organic compounds which can produce dangerous compounds, known as disinfection byproducts (DBPs).  Wastewater treatment with Chlorine may produce Aromatic organic halides (AOX) [3], Trihalomethanes (THMs), Dioxins. Skin diseases and inhalation problems also major concern.  Most of the time this process can not maintain discharge standard.

 


2.  Methodology: 
 This paper is based on investigating selective factories equipped with different types of ETPs. Samples were collected from different sampling points of ETPs. Samples were analyzed for various physicochemical parameters like pH, total dissolved solids (TDS), total suspended solids (TSS), 5 days biochemical oxygen demand (BOD5), chemical oxygen demand (COD), Dissolved oxygen (DO) etc.  This study includes test report of Environmental Engineering Laboratory (BUET), Department of Environment (DoE) and Department of Applied Chemistry & Chem. Tech. (DU).  Suggestions of ETP executives, operators and designers are also included in this paper.

3.  Selection criteria of textile effluent treatment plant:[4]
Textile wastewater treatment techniques vary greatly in their strength and compatibility. Following are some application criteria, which are normally relevant in evaluating the right selection of ETPs:
      -  Flow rates, composition and concentration of influent.
      -  BOD and COD  removal efficiency.
      -  Operation and maintenance cost (O&M).
      -  Sludge production.
      -  Sludge treatment & disposal cost.
      -  Performance in winter and summer.
      -  Performance in high and low temperature water.
      -  Expandability.
      -  Ease of installation.
      -  Energy efficiency.
      -  Initial investment and space requirements.
      -  Upset recovery efficiency.

5.  Why biological treatment is the best method?
      1.  Economic perspective.
     2.  Efficiency perspective.
     3.  Ecological perspective.

5.1.  Economic perspective: Effluent treatment with biological method is very cost-effective compared to other methods because of least operation and maintenance cost due to very low chemical consumption, low labor cost, less sludge treatment and disposal cost. It will be clearly realized by following comparative cost analysis tables.



* maintenance cost and sludge disposal cost has been neglected.


Generally in combined method-1 (Fig:3) chemical treatment (coagulation & flocculation) is  done   before biological treatment. By modifying (first biological then chemical treatment, Fig:4)   running cost per m3 is reduced 28% due to less chemical cost, sludge treatment and disposal cost.

      
      
       5.2.  Efficiency  perspective:
  Efficiency of effluent treatment plant is estimated by the reduction percentage of BOD, COD, TSS, TDS, Color and other parameters like chloride, phosphorus, nitrogen, phenolic compounds etc. Biological effluent treatment plant is highly efficient. Table 4 exposes the comparative degree of efficiency of different active ETPs.

               Standardization:

 

Factory type

Composite textile industries.

ETP treatment capacity

55 - 150  m3/hour

Avg. Inlet BOD5 range

110 – 281  mg/l

Avg. Inlet COD range

284 – 480  mg/l

Avg. Inlet TSS range

62 – 276  mg/l

Avg. Inlet TDS range

240 – 4950  mg/l

Avg. Coagulant dose (FeSO4)

583 – 984  mg/l

Avg. Coagulant dose (Lime)

330 – 606  mg/l

Avg. Inlet PH

6.7 – 11.5

Avg. Inlet temperature

35 – 500C

Discharging area

Inland surface water.

 

 

 

          Courtesy: Beximco Textile Ltd, Padma Pollycotton Ltd, DBL Textile Ltd, Interstoff  Apparel Ltd, Texurop (BD) Ltd. and rests  are confidential.

From the above performance analysis table 4 , we can figure out the actual scenario of different active ETPs.  It is very clear that there are wide variations in average removal efficiency compared to typical efficiency of ETPs.  Now take a look at individual efficiency discrimination of important parameters based on performance analysis table 4.

  1. Biological oxygen demand (BOD5):

  
 - From figure 5 we can see that except chlorination based ETP  D1, other BOD5  levels at discharging points are under maximum permissible limit (MPL).

 - In biological method the average BOD removal efficiency  gained the highest value(84%) compared to other methods.


  2.  Chemical oxygen demand (COD):

  
   -  In combined  bio-chemical method the average COD removal efficiency  gained the highest value(70.8%) , in biological method 59.1%.

 3.  Total suspended solids (TSS) :

 
  - Among all methods highest average TSS removal efficiency (81.7%) found in biological method.


 4.  Total dissolved solids (TDS) :

 - From figure 8  it is found that before treatment TDS level was under discharging standard (2100 mg/l)  in   ETPs A1,A2,C1,C2,C3,D2,D3.

 - Physico-chemical based etp A3 and chlorination based etp D1 can not maintain discharging standard.

 - It is also found that except biological method, TDS value increased after treatment in Physico-chemical based etp A2, combined  bio-chemical etp C3 and chlorination based etp  D2,D3.

 - Biological treatment reduces TDS significantly and satisfy discharging standard.

5.  Average removal efficiency (RE%) :


 - Figure 9 shows that among four methods, highest efficiency in BOD  removal (84%)  is  obtained by biological treatment.                                                                                                   

 - Combined physico-chemical and biological method reduce COD very efficiently.                     

- Highest average  efficiency in TSS  removal (81.7%)  is obtained by biological treatment.       

 - Biological treatment can reduce TDS with high efficiency. Rest of the methods mostly increase TDS  after treatment. Great increase in TDS and zero efficiency by oxidation method with chlorine can be realized from figure 9.


 6. Dissolved oxygen (DO) and residual chlorine (RCL):
-  From table 4 it is found that, DO level of treated effluent is less than discharge standard (4.5- 8 mg/l)  in combined bio-chemical method (C1 & C2) and chlorination method (D2). Chlorine based etp D3 presents higher DO (8.5 mg/l) than permissible limit. Lower DO could be a threat to aquatic life. Organisms undergo stress at reduced DO concentrations that make them less competitive to sustain their species within the aquatic environment.

- In chlorination method residual chlorine is found much  higher than permissible limit (0.3 mg/l) which can  react with natural organic compounds and produce dangerous disinfection byproducts (DBPs).

Result Summery of efficiency analysis:
By physico-chemical process BOD and COD removal efficiency was  found 50% and 70%  respectively(Nicolaou and Hadjivassilis,1992), BOD and COD removal efficiency was found 90%  and 75%  respectively by biological treatment [9], In combined biological and physico-chemical method BOD,COD removal efficiency exceeded  90% [7]. Treatment with chlorine gained 55-85% BOD, 55-70% COD reduction efficiency [8]. But analyzing average efficiency of different active ETPs from table  4, it is realized that the actual view is quite different.  Physico-chemical as well as highly efficient combined physico-chemical and biological  methods can not satisfy discharging standard. Due to high chemical consumption, labor cost, huge quantity toxic sludge disposal problems owners do not run the plant regularly and efficiently. Sometimes it’s just for eye wash. It is also found that, treatment capacity of certain ETPs is lower than incoming effluent stream.  So, huge quantity wastewater is discharged without any treatment.  But owners are bound to run biological plant 24 hours and  365 days to ensure that the bacteria are provided with sufficient “food” (i.e. wastewater) and oxygen to keep them alive.  Brief breaks (for a few hours) in operation will probably do little harm but prolonged shut down will deprive the microorganisms of their food and oxygen and will damage the process [3]. That’s why in actual practice biological plant is running so efficiently maintaining national standard. 

  5.3.  Ecological perspective:

Biological treatment with activated sludge process (ASP) using aerobic  bacteria is very eco-friendly. No hazardous chemicals are required, so probability of water contamination is nearly zero. No bad smell is produced.  Possibility of reusing treated water for irrigation and fish life which is impossible in other methods. Very low non-toxic sludge is produced containing negligible amount of toxic heavy metals like lead, chromium, mercury etc.  Sludge can be utilized as compost fertilizer.     

Courtesy:  TEXMAC  (BANGLADESH)  LIMITED.

Table 5:  Sludge characteristics.

             Parameters

Physico-chemical

Biological

Combined
 bio- chemical

Chlorination

Sludge quantity

2–5 kg/m3

300 – 400 gm/m3

2–5 kg/m3

Negligible

Sludge toxicity

Highly toxic

Non-toxic

Toxic

-

Sludge disposal problem

Severe

Slight

Medium

-

Sludge disposal cost

High

Very low

High

-

Sludge utilization

Brick

Fertilizer, brick

Brick

-

   

Problems with Chlorination:

Breathing small amount of chlorine gas can be deadly. It is toxic to mucous membrane, damage respirational systems, coughing, chest pains, fluid accumulation in lungs. Moreover, chlorination of effluents such as those from dye houses may produce cancer causing chemicals due to the reaction of chlorine with aromatic chemicals in the effluent.  These chlorinated organic chemicals are part of a group of chemicals known as AOX (aromatic organic halides), and are undesirable. It is therefore recommended not to use chlorine in an ETP that treats textile dyeing wastewater [3]. 


 6.  Scopes of optimizing effluent generation & toxicity:[5]
Both toxicity and volume of textile effluent is directly related to the cost of operation of the effluent treatment plant. Following attempts can be practiced to minimize effluent generation and toxicity:
-  Bleach and Salt bath recovery.
-  Dye substitution-Use low-salt reactive dye and metal free dyes with high fixation rate.
-  Low liquor ratio dyeing machines. Ex: Thence jet flow dyeing (HK). m:l-1:5
-  Cold pad batch dyeing.
-  Recycling/reuse of cooling/condensate water.
-  Pulsating rinse technology.
 - Use biodegradable surfactants such as linear alcohol ethoxylate.
 - Caustic scouring is responsible for 54% of total BOD, 49% of total COD, 10-20% of total pollution load. So replace caustic scouring with bio-scouring.
 - Replace the use of chlorites and hypochlorites with hydrogen peroxide.
 - Right-first-time dyeing.
 - Replace chlorinated solvents with unchlorinated alternatives.
 - Organic salt so called CLR is useful to consume half amount of Gluber salt or sodium chloride.
 - Use enzyme based H2O2  killer instead of salt base H2O2  killer.
 - Utilize flow segregation system to reduce pressure on biological treatment unit [6].


 7.  Constraints of Biological ETP :

 -  Non biodegradable dye stuffs render the biological treatment ineffective or less efficient.
 - Presence of excessive toxic heavy metals prevents microbiological growth, hence efficiency fall down.
  - Biological treatment is less effective in high osmotic pressure due to high TDS content in waste water.
 - Higher space required.
 - Initial investment is higher than other system due to constructing bigger civil works and electromechanical equipments.
 - High retention time (12 to 72 hours depends on COD level, COD ≤1000 mg/l need 12 to 24 hours, COD 1500-2000 mg/l required 72 hours typically).
 -  Low decolorization efficiency of soluble dye stuffs.

8.  Suggestions:

- DO concentration needed for aerobic system 1.5 to 4 mg/l. Generally 2 mg/l is maintained.  Higher DO will not increase biodegradation efficiency hence represent energy wastage.     Noted that, aeration possesses 25%  of total running cost.
- MLSS concentration should be maintained within the range of 1500-3000 mg/l for activated sludge method.
 - PH should be maintained between 6.5-8.5 and temperature less than 400C in biological reactor.
 - Sludge settling characteristics should be checked by SVI (sludge volume index) test.  Poor settling sludge will result in low concentration of solids in the return activated sludge thus the concentration of microorganisms drops and subsequently F/M (food per microorganism) ratio increases in the aeration tank which results in a reduced BOD,COD removal efficiency. For excellent sludge settling SVI value should be less than 50.
 - Add adsorbents like bentonite clay or activated carbon to biological system in order to
  eliminate non-biodegradable or microorganism–toxic organic substances (Pala and Tokat, 2002).
 - Higher microbial efficiency can be obtained by adding nutrient salts like Urea,
   Diammonium phosphates (DAP). Nutrients (food) should be provided while plant shutdown.

9.  Conclusions:  
This study revealed that textile effluent treatment with biological methods is highly efficient and cost-effective as well as eco-friendly.  Though there are some constraints specially, extremely high initial investment and space requirements which are major obstacles for small and medium scale factories.  Government can take efforts to initialize bank loan to establish effluent treatment plants with minimum interest.  Further development is essential to treat inorganic compounds by biological process.

References:
   [1]  Sen, S. and Demirer, G.N. Anaerobic treatment of real textile wastewater with fluidized   bed reactor. Water Researce 37,2003: 1868-1878.
   [2]  Khan M, Knapp J, Clemett A and Chadwick M, Managing Pollution from small industries           in Bangladesh, Technical report, Researce for Development, Department for international Develpoment (DFID), 2006.
   [3]  Mohidus Samad Khan, Knapp J, Clemett A, Chadwick M, Mahbub Mahmood, Moinul Islam Sharif, Managing and Monitoring Effluent Treatment Plants, Page-3,5,7. 

   [4]  Thomas E. (Tom) Schultz, Biological wastewater treatment, October 2005, page 3.
   [5]  Tanveer Ahmed, Shafi M Tareq, Textile industries in Bangladesh-A rising environmental   degradation down the drains, Bangladesh Textile Today,issue 1,January 2008,            http://deletionpedia.dbatley.com/w/index.php?
   [6]  Mohidus Samad Khan, Knapp J, Clemett A, Chadwick M, Improving effluent treatment           and management. Page 7-8.
   [7]  Samiya Ahmed, Alexandra Clenett, Matthew Clark, Kelvin Tapley, Choosing an Effluent Treatment Plant, page-29,30.
   [8]  A. K. M. A. Quader, Treatment of textile wastewater with chlorine: an effective method,        3 May 2010.
   [9]  Adebayo, G. B, Otunola, G. A and Ajao, T. A, Assessment and biological treatment of effluent from textile industry, 19 June, 2009

About the author:

Md. Rashaduzzaman  Mithun is the champion of Textile Talent Hunt 2010. The article was his project work during the competition. Currently he is working in the industry.



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Bangladesh Textile Today
Issue: March , 2012