Separation and recovery of fats in tannery wastewater

• 1. Effluents in the tanning industry and their impact on the environment.
• 2. Coagulation-flocculation using a DAF for oil recovery and effluent clarification.
• 3. Treatment study in Sigmalab laboratory.
• 4. Conclusions

1. Effluents in the tanning industry and their impact on the environment

The leather tanning industry, especially leather, contributes enormously to the industrial growth of countries and to the global economy, being one of the most important industries, especially in countries such as India, Bangladesh, China, Pakistan, Turkey and Brazil.

The leather production process consists of batch-operated unit processes that require large quantities of chemicals that are hazardous to the environment and living organisms (plants, animals, and humans) and highly polluting to aquatic environments. Some of the most commonly used products are: sodium hydroxide (NaOH), sodium chloride (NaCl), pentachlorophenol (C6HCl5O), sodium sulfide (Na2S), enzymes, lime slurry (Ca(OH)2), chlorides and free chlorine, sulfuric acid (H2SO4), chromium, formic acid (CH2O2), ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), metal salts, and organic chemicals.

In addition to these products, water consumption in this industry is among the highest, with over 40 m³ used per ton of raw material processed. Details on the tanning process and effluent pollution can be found in the following article published on the SIGMADAF Technology Blog.

Treatment processes focus on two objectives: purification of wastewater and recovery of raw materials. The water is treated to make it suitable for discharge into the general sewage system, or even, in highly intensive processes, for discharge into the aquatic environment or for secondary uses. The raw materials that can be recovered during the treatment stages are: collagen, collagen gels and fibers, fats, keratin, polypeptides and proteins.

2. Coagulation-flocculation with DAF for fat recovery and effluent clarification

Among all possible treatments and complete processes to treat and manage tannery effluent, physicochemical treatments (acidification or alkalinization, emulsion breaking and coagulation-flocculation) combined with DAF flotation are an indispensable part, not only for primary effluent treatment, but also for the recovery of raw materials such as fats and other compounds.

Coagulation is the destabilization of colloidal compounds, fats, suspended solids and solids and organic matter susceptible to precipitate from the liquid phase, by neutralizing the forces that separate them and the interaction of charges that allow the compounds to pass into the solid and separable phase. The particles collide to form larger particles, these collisions must be promoted by the use of a second chemical: flocculant polyelectrolytes.

It has been demonstrated that the use of inorganic coagulants, in combination with a polyelectrolyte flocculant, allows the effective elimination of grease, organic matter and heavy metals, especially chromium (III), a metal that is highly polluting in aquatic environments and harmful to the health of organisms.

The most widely used coagulants are the following:

• Aluminum sulfate
• Aluminum polychloride
• Ferric chloride
• Ferrous sulfate

The coagulation-flocculation process can be assisted with emulsion breaking steps by applying special chemicals for this purpose: changing the pH by alkalinization or acidification and using products specially designed for tannery fats.

Aluminum polychloride is identified as the most effective coagulant. This coagulant, along with other specific chemicals, will be used in the grease treatment and recovery study carried out in Sigma's laboratories, which is described below.

A properly designed physicochemical treatment with DAF flotation allows primary water treatment and grease recovery.

3. Treatment study at the Sigmalab laboratory

Laboratory-scale tests and studies allow the identification of specific chemicals for effluent treatment, the doses and the hydraulic residence times required for each reaction. The analysis of the clarified products allows to establish the purification performance that can be achieved.

SIGMA's laboratories receive four effluents from different stages of a tannery process: A) first wash or riverbank, B) second wash or riverbank, C) calcining and D) macerating. Each of these effluents has specific characteristics and requires independent chemical treatment to separate the fats and clarify the water. Subsequently, all clarified effluents will be unified and treated in a common line.

Please visit our website to learn more about our laboratory services, and feel free to contact us if you have any questions about this work or would like to request testing of your effluents.

A. Treatment of the effluent from the first wash or riverbank.

Effluent with a milky and apparently homogeneous appearance, requires treatment combining an activated carbon adsorbent, followed by a PAC coagulant, pH adjustment and a cationic flocculant. Solids separation occurs with the addition of coagulant and an increase in pH to 8-9. In addition, grease separation is also promoted by raising the pH. A high retention time is required at each step so that each chemical has sufficient time to react with the water. Fats and solids are removed in the form of consistent flocs.

The analytical results are as follows: high removal of organic matter and suspended solids and recovery of almost half of the fats.

	Sample A	Clarified	Performance
pH	7	8 - 9
Fats	292	157	46%
Total organic carbon	2750	2270	17%
COD	15000	4500	70%
DBO5	6060	1520	75%
Biodegradability		34%
Suspended solids	4570	912	80%

B. Treatment of the effluent from the second wash or riverbank.

This apparently homogeneous, milky-looking effluent requires a treatment that combines an acid shock with sulfuric acid followed by fat separation using an emulsion breaker. The pH is first reduced to extreme acidic conditions and then an emulsion breaker is applied, immediately observing grease separation. The fats and oils are soft and fluffy, and separate and float away leaving the sample to stand for over an hour.

The analytical results are as follows: very high removal efficiency of fats, solids and organic matter.

	Sample B	Clarified	Performance
pH	8 - 9	3
Fats	2140	140	93%
Total organic carbon	13700	1640	88%
COD	81200	6210	92%
DBO5	36900	3440	91%
Biodegradability		55%
Suspended solids	15700	520	97%

C. Treatment of the calcining effluent.

This effluent is oily and greasy in consistency, dark in color, with visible solids and "chunks" that separate naturally, but not enough to originate a quality clarified, the pretreatment is a mechanical coarse separation followed by an acid shock with sulfuric acid and followed by a separation of fats by an emulsion breaker. For this sample, a first separation of settleable solids is required, allowing the coarse solids to precipitate and then mechanically extracting the liquid phase. Subsequently, an acid shock is applied. The pH is initially reduced to neutral pH (starting from a sample value of 13) and, as fat separation is immediately observed, an emulsion breaker is applied. The fats and oils separate easily into compact, oily lumps.

The analytical results are as follows: high fat removal efficiency and moderate organic matter removal efficiency.

	Sample C	Clarified	Performance
pH	13	7
Fats	474	98.4	79%
Total organic carbon	10700	5500	49%
COD	54900	42600	22%
DBO5	36200	16200	55%
Biodegradability		38%

D. Macerate effluent treatment

This effluent has the appearance and consistency of "whey", whitish and viscous, the treatment is direct with PAC coagulation followed by flocculation with a cationic polyelectrolyte. Solids separation occurs immediately with the addition of coagulant, then the flocculant agglomerates the coagulated material. High reaction times are required.

The analytical results are as follows: moderate removal of organic matter, solids and fats are not very high in the sample, so the treatment has little influence on them.

	Sample D	Clarified	Performance
pH	8 - 9	8
Fats	42.8	40.8	5%
Total organic carbon	2675	2180	19%
COD	10500	7830	25%
DBO5	4340	2700	38%
Biodegradability		34%
Suspended solids	236	101	57%

4. Conclusions

• The industrial process of leather and fur production is one of the largest industries in the world, contributing enormously to the economy of countries. But it is also one of the largest consumers of water and generators of wastewater, which is highly polluting and dangerous for the environment and organisms.

• Wastewater from the tannery process is rich in hazardous chemicals, solids, grease and organic matter. It requires extensive treatment consisting of effluent pretreatment and conditioning, biological treatments and polishing stages using membranes and other advanced technologies. The treatment objectives are water purification and recovery of raw materials such as fats and greases.

• At SIGMALAB laboratories, a pretreatment study is carried out on the effluents from four tannery process lines: two lines of washing, calcining and macerating. Each effluent must be pre-treated independently as it requires different chemicals. The clarified effluents from these four pretreatments will be unified for subsequent joint treatment.

• Pretreatments consist of physicochemical processes that require acids, bases, emulsion breakers, coagulants and flocculants. The effectiveness of contaminant removal is high. Separated flocs and fats can be floated in Sigma DAF equipment for removal and/or recovery into products of interest.