Printing and Dyeing Effluent: Why COD Keeps Climbing and What a Reuse Train Must Strip Out

Eight streams behind a single outfall

The printing and dyeing industry is a major industrial wastewater discharger. Incomplete statistics put national discharge at 3x10^6-4x10^6 m3 per day, and the water is high in volume, high in organic pollutant content, deeply coloured, strongly alkaline and highly variable in quality. Treating it as one uniform stream is the first mistake.

Desizing wastewater is low in volume but concentrated, carrying sizing agents and their decomposition products, fibre debris, acids, starch, alkali and enzymes, with high turbidity and pH around 12. With starch sizing agents both BOD and COD are high and biodegradability is good; with synthetic sizing agents COD is very high while BOD is below 5 mg/L and biodegradability is poor. Scouring wastewater is high in volume and concentration, carrying cellulose, pectic acid, waxes, oils and fats, alkali, surfactants and nitrogen compounds, strongly alkaline and hot, brown, with COD and BOD reaching several thousand milligrams per litre. Bleaching wastewater is large in volume but lightly polluted, with residual bleaching agent and small amounts of acetic acid, oxalic acid and sodium thiosulfate. Mercerising wastewater carries NaOH at 3%-5%, though most mills recover it by evaporation and concentration, so what finally leaves after repeated reuse is still strongly alkaline with relatively high BOD, COD and SS.

Dyeing wastewater is the most variable, sometimes carrying sodium sulfide, tartaric emetic, aniline, copper sulfate and phenols that come with particular dyes. It is alkaline with pH sometimes above 10 when sulfur or vat dyes are used, intense in colour, low in SS, and has COD higher than BOD with poor biodegradability. Printing wastewater carries sizing agents with high BOD and COD. Finishing wastewater is small in volume but contains fibre debris, resins, formaldehyde, oils and sizing agents. Alkali-reduction wastewater, from polyester imitation-silk processing, carries polyester hydrolysates such as terephthalic acid and ethylene glycol, with terephthalic acid content as high as 75%, pH generally above 12, and CODCr from the alkali-reduction step reaching 90,000 mg/L. It is a high-concentration, refractory organic waste.

Why so many plants still get it wrong

Successful cases exist, and so do plenty of disappointing ones, usually for the same reasons. A plant copies another plant's experience without analysing its own wastewater quality and quantity. Municipal design codes are applied with a few parameters changed, which caused heavy losses when large centralised plants were handed to design institutes that did not understand the water. New technology, processes and chemicals go straight into full-scale projects without pilot testing; pilot scale is typically 3%-5% of project flow, meaning a scale-up of at most about 20 times, and taking laboratory results directly into a project rarely succeeds.

The water itself has also moved. Imitation silk and advances in finishing brought poorly biodegradable organics into the stream: PVA sizing agents, rayon alkali hydrolysates, mainly phthalate compounds, and new auxiliaries. COD concentrations have risen from a few hundred mg/L to 2,000-3,000 mg/L, pushing COD removal by existing biological treatment systems down from 70% to around 50% or even lower. Conventional chemical precipitation and flotation manage only about 30% COD removal on this kind of water.

Coagulation as the front end

Coagulation doses chemicals so that most water-insoluble dye particles and colloidal organics agglomerate into larger particles, removed by natural sedimentation, flotation or similar means. The flocs adsorb strongly, so a portion of water-soluble organics goes too. Coagulation removes more than 80% of suspended organic pollutants, with colour removal of 50-95%. The key is choosing the right flocculant. Aluminium sulfate, ferric sulfate and ferric chloride work well on wastewater carrying water-insoluble dyes such as disperse, vat and sulfur dyes, where COD and colour removal are both high.

A representative train runs process wastewater to an equalisation tank, then hydrolytic acidification, biological contact oxidation, an intermediate settling tank, a coagulation reaction tank, a flotation tank and out. Treatment chemicals are polyacrylamide and polyaluminium chloride, with calcium hypochlorite as the decolourising agent. Aerobic biological contact oxidation is the heart of the process and degrades the organics. Coagulation generally calls for anionic and non-ionic polyacrylamide while sludge dewatering calls for cationic polyacrylamide, and a cationic grade with relatively low charge density and a molecular weight above 10 million usually performs best.

Practice varies by country. In the United States most printing and dyeing wastewater gets secondary treatment, physicochemical pretreatment plus biochemical treatment, with a few enterprises adding activated carbon adsorption as a tertiary stage. Japanese enterprises run similar processes with somewhat more ozonation. In China, physicochemical treatment is combined with a second specialised chemical stage; coagulation-sedimentation and coagulation-flotation dominate the physicochemical side, while most biochemical facilities use the activated sludge process with SBR applications increasing steadily.

Discharge limits and the reuse gap

The discharge standard is the Discharge Standards of Water Pollutants for the Textile Dyeing and Finishing Industry (GB 4287-1992). Category I covers harmful substances that accumulate in the environment or in animals and plants and have long-term effects on human health; Category II covers substances whose long-term effects are smaller. Listed maximum permissible discharge concentrations include 0.05 (as Hg), 0.5 (as Cr6+), 1 mg/L (as Cu), 5 mg/L (as Zn), 10 mg/L (as F), biochemical oxygen demand measured over 5 days at 20 °C, chemical oxygen demand by the potassium dichromate method, and cyanide (as CN-).

The sector is a heavy water user. Statistics show that in 2008 the textile industry discharged 2.3 billion tonnes of wastewater, third among all industrial sectors and 10.60% of national industrial wastewater discharge, while CODCr discharged came to 314,000 tonnes, fourth among all sectors and 7.76% of the national total. Those figures only cover enterprises above a designated size, and since SMEs account for 99.6% of the industry and non-public enterprises 95%, the real numbers are higher. If 70% of fibre processing volume requires printing and dyeing, annual discharge runs to about 3 billion tonnes. The state required a 60% reuse rate during the 11th Five-Year Plan; the post-treatment reuse rate has not even reached 7%.

Closing the loop

Reuse is where chemistry and filtration have to meet. In one common scheme lime serves as the pH adjuster and ferrous sulfate as the coagulant, so effluent iron content is high and the water cannot be reused directly. Intensified aeration in the contact oxidation tank converts all ferrous iron to ferric iron, forming ferric hydroxide microflocs that make the effluent turbid and coloured. Adding polyaluminium chloride and PAM binds those microflocs into larger flocs, and high-efficiency filtration removes the iron.

That filtration step is often an AFF asymmetric fibre filter, a unit integrating dosing, microflocculation, sedimentation and filtration. It filters at more than 10 times the rate of sand filtration with a precision of 5 μm, four times that of ordinary sand filters, backwashes easily and is convenient to manage. After AFF filtration the reclaimed water still has a COD of around 100 mg/L, mostly soluble COD (SCOD), which undermines reuse value and shortens reverse osmosis membrane life, so a suitable process must bring COD below 30 mg/L.

A membrane biological fluidised bed (MBFB) process is therefore adopted, combining a specially treated ceramic membrane separation system with a high-load biological fluidised bed. It has already been promoted in the United States, Japan, the United Kingdom, Germany, South Africa and Australia. Apart from conductivity, MBFB-treated effluent meets the industry standard for workshop reuse water and can be used directly in washing, soaping and rinsing workshops, giving a reuse rate of roughly 60%. It can also feed RO membranes directly for desalination without complex cartridge filtration and ultrafiltration stages, cutting pretreatment cost and extending membrane life. Underpinning all of it: comply with environmental regulations, choose a mature process that runs simply and cheaply, keep the flow short and shock-resistant, allow for sensible pipe layout, and make sure the treatment station produces no secondary pollution.