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28 June 2021

Treatment of wastewater from the tanning industry. Sigma DAF technologies application.

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1. Introduction: tannery and wastewater

Photo of several samples of processed and treated animal skins on a work table, for the industrial production of leather articles.

Today, the production and trade of leather presents a very strong economic activity in the world, constantly dynamic and increasing the movement of exports. The main markets are in Spain, Mexico, Korea and the United States, representing around 75% of total export movements in the world.

 Among them, Spain is the one with the highest growth, with more than 250 industries, of which 60% are located in Catalonia and 35% in Valencia, Murcia and Madrid

The European tanning industry consumes around two million tonnes of animal skin and hides per year.

TANNERY is the process of treatment and transformation of the skin of various animals into LEATHER. This process allows to avoid deterioration due to environmental conditions and the degrading action of microbes, fungi, insects and other microscopic life forms

The process consists mainly of adding a series of tanning products to the hides: chromium salts and/or vegetable tannins. These products penetrate the skin and fixate on its structure, blocking chemical and biological degradation reactions, increased by humidity, obtaining an inert and resistant structure to these processes.

After separating the skin of the slaughtered animal, it is generally treated with salt to avoid putrefaction and to be preserved until the moment of its processing. Tanning is carried out in a sequence of stages, discontinuously, in which water consumption is very high, generating polluting gases, polluted liquid effluents and solid waste, the liquid effluent being the most polluting and abundant.

The four main steps of tanning process are:

  • Preparatory steps: soaking, liming, de-fleshing, de-liming, bating, pickiling.
  • Tanning: Chromium, vegetable, aldehyde or synthetic tanning.
  • Post-tanning: samming, splitting, skiving, neutralizing, greasing, drying, coloring and finishing.

The two main wastes generated in the tanning processes are SOLID WASTE and WASTE WATER.


Solid waste:

These wastes consist of salt, shavings and leather trimmings, untreated sludge from the wastewater treatment plant, remains of chemical products and their packaging, etc. They are products that are NOT recoverable (with the exception of sludge after adequate treatment as will be described below), and must be disposed in controlled landfills, which are another environmental problem.

Untanned by-products (raw skin trimmings, hair, wool, grease and sebum, meat, etc.) can be derived for industrial applications such as gelatin, glues and collagen manufacturing.

Wastewater:

The water consumption is between 25 - 60 m3 per ton of fresh skin, and the yield is 500 kilograms of finished leather per ton of fresh skin. Depending on the process, unique to each industry, more than 440 kilograms of chemical products can be used per ton of fresh skin. These chemicals are dissolved in process water, which generates an effluent with a very high content of organic load and contaminating inorganic compounds (chromium, chlorides, ammonium, sulfides, sulfates).

There are two options for collecting and treating wastewater: i) individual treatment of wastewater from each of the stages, separately, and specially designed depending on whether it comes from preparatory, tanning or post-tanning; ii) mixing and homogenizing all the discharges of each stage and joint treatment of the global wastewater, this is the most commonly used option since it involves the installation of a single water treatment plant instead of one plant per discharge.

2. Tannery wastewater characterization

Typical wastewater from tanneries has a very high concentration of organic load (COD) most of it being biodegradable, and high nitrogen content, since the skin is made up of proteins, keratins, fats, etc. In addition, organic compounds (tanning agents, synthetics, fats, colorants, etc.) are applied in the tanning process.

Water also contains inorganic compounds (chromium, chlorides, ammonium, sulfides, sulfates, etc., among which chromium is the one with the highest concentrations and problematic) and a very high salt content (in the tanning process salts are used to the preservation of the skin, sulfides, chromium salt, etc.) which results in high levels of alkalinity, with a pH of around 10, of a mixed and homogenized wastewater. The oil and fat content is usually depreciable

The characteristic composition of a homogenized wastewater from a tanning industry is as follows:

Table 1. Composition of homogenized effluents from the tanning industry. Adapted from Artiga 2005.

Parameter (mg/L)

Common efluent values

Effluents previously subjected to hair and chromium recovery and desulfurization

COD

7000 - 8000

5000 - 5500

BOD5

4000 - 4500

3000 - 3500

NH3

200 - 250

<200

SS

3500 - 4000

2500 - 3000

Cr3+

200 - 300

80 - 100

S2-

200 - 250

<2

SO42-

1800

1800

Cl-

5000 - 6000

5000 - 6000

The characteristics of each pollutant in this wastewater can be summarized as follows:

  • Salts: salts and the high conductivity they cause are not easily removable and make biological treatment difficult when they are in excess.
  • Organic matter: It is found in very high concentrations, being mostly biodegradable. Properly designed biological treatment with effective pre-treatment can achieve removal of up to 99% of COD.
  • Sulphides:  can be removed by an oxidation process as part of the pre-treatment.
  • Total nitrogen and ammonium nitrogen: they come from the leather and ammonium-based products added during the tanning process. They are eliminated in a biological treatment by nitrification-denitrification.
  • Chromium: it is found mainly as Cr3+. Generally, it does not affect biological treatment, but its recovery is recommended in the pre-treatment. The main challenge of the pre-treatment design is to select an appropriate technology for the reduction of chromium in the effluent.

In case that a separation of the wastewater of each tanning stage is designed, the treatment must be adapted according to the typical characteristics of each one, in addition this separation facilitates the treatment and increases the recovery performance of components of interest and value such as it's the chromium. However, this method is not frequent given its management cost compared to treating homogenized wastewater.

These characteritics variate as indicated in the following table:

Table 2. Characterization of the effluents from each stage of the tanning process. Values are shown based on tons of raw cow skin. Adapted from Artiga 2005.

 

Preparatory

Tanning

Post-curtición

Total

Volume (m3/ton)

20 - 25

1 - 3

3 - 8

24 - 37

COD (kg/ton)

120 - 160

10 - 20

15 - 40

145 - 230

BOD5 (kg/ton)

40 - 60

3 - 7

5 - 15

48 - 86

SS (kg/ton)

70 - 120

5 - 10

10 - 20

85 - 155

Cr3+ (kg/ton)

-

2 - 5

1 - 2

3 - 7

S2- (kg/ton)

8 - 12

-

-

8 - 12

Nitrogen (kg/ton)

10 - 20

-

1 - 2

11 - 22

Chloride (kg/ton)

120 - 200

50 - 60

5 - 10

175 - 270

Sulphates (kg/ton)

5 - 20

30 - 50

10 - 40

45 - 110

3. Efficient treatments of tannery wastewater

The processes most used in tanning water treatment are PHYSICAL-CHEMICAL PROCESSES and BIOLOGICAL TREATMENTS, and currently the trend is towards the use of MEMBRANE TECHNOLOGIES to allow the reuse of treated water in the process.

The inorganic compounds present in the wastewater can have toxic and inhibitory effects for the microorganisms of the biological treatment, for which a physical-chemical pre-treatment is carried out for their elimination.

When streams are not separated but they are mixed and homogenized, the following points are the basis of treatment design:

  • High chromium content (mostly as Cr3+ although it can occasionally be found as Cr6+). This reduces the possibility of reusing and revaluing the sludge generated in the biological treatment.
  • High concentration of organic load. If this load is too high and the biological treatment does not reach the required performance, a post-treatment stage must be installed (generally membrane technologies are applied).
  • High content of sulphates. This makes the application of an anaerobic biological treatment not recommended. An alternative consists of the application of desulfurization technologies included in the pre-treatment.
  • High solids content. Proper pre-treatment design can significantly reduce solids. The coagulation-flocculation and primary clarification processes (decanters or DAF -dissolved air flotation- equipment) are highly effective in removing solids. An adequate pre-treatment design allows the process to be protected against these solids and a considerable reduction in the volume of influent to the biological treatment, with the savings in equipment, space and energy that this implies.

If, in addition to carrying out the wastewater treatment, it is desired to use the final clarified water in reuse, the following points must be considered:

  • Chromium recovery during pre-treatment. A properly designed pe-treatment for this purpose can remove 95-100% chromium from the water. If the chromium is removed, the sludge generated during the biological treatment can be applied as fertilizer after further conditioning.
  • The installation of an anaerobic biological treatment allows the generation of biogas. If installing this type of treatment is desired, sulphides and sulphates have to be previously eliminated. After the anaerobic treatment, an aeration treatment must be installed to achieve the elimination of organic load that meets the discharge limits.

The PHYSICAL-CHEMICAL PROCESS consists of the treatment of the homogenized effluent by adding precipitating agents for the elimination of chromium (sodium hydroxide NaOH), coagulants (iron chloride FeCl3 or aluminum salts) and flocculants (polyelectrolytes).

3.1. Physical-chemical treatments for the pre-treatment

The pre-treatment of wastewater from the tanning industry generally consists of physical-chemical processes, which allows the removal of solids, sulphides, chromium, etc. in order to improve the efficiency of the subsequent biological treatment.

The most used operations are the following:

Homogenization: uniform mixing of the wastewater streams from the various stages of the tanning process. This allows a constant flow rate and concentrations of the influent to the treatment.

Screening: physical separation of larger thicknesses by means of screens and sieves.

Sulphur removal: oxidation with air or other more powerful oxidants is applied. Oxidation with air is slow and requires a catalyst for its acceleration, it is carried out at pH around 11; sulphide removal efficiencies can be achieved with concentrations below 1 mg/L in the effluent. Other agents applied as strong oxidants are: hypochlorous acid HClO, chlorine Cl2, hydrogen peroxide H2O2 and ozone O3.

Coagulation – flocculation: in addition to solid particles that settle naturally, there are colloidal particles and particles that do not settle easily, due to their electrostatic charge, they repel each other and remain in suspension. Coagulants are applied to destabilize this charge so that suspended and colloidal particles can precipitate.

Occasionally, a polyelectrolyte can be applied as a flocculant to agglomerate these precipitated particles and form flocs with greater sedimentability or buoyancy, which will allow the optimization of the subsequent clarification stage. In addition, this precipitation allows the reduction of chromium, sulfates and organic matter associated with separated solids, which mainly consists of slowly biodegradable material that would make biological treatment difficult.

The removal rates that can be achieved are >60% COD, >55% BOD5, >30% sulphates and >95% chromium. The most widely applied coagulating and precipitating agents are currently the aluminum and iron salts, mainly iron chloride FeCl3, aluminum chloride AlCl3 and aluminum sulfate Al2(SO4)3 and precipitating bases such as calcium hydroxide Ca(OH)2 or hydroxide of sodium NaOH. Chromium removal with these precipitating and coagulating agents requires a controlled pH of around 7.5.

In some tanning processes chromium is not applied, but tannins are applied. These substances are hardly biodegradable and are eliminated in the pre-treatment by precipitation with aluminum or iron salts, the COD can be reduced by approximately 50%.

Solids separation by flotation: dissolved air flotation or DAF systems are widely used in the pre-treatment of wastewater from the tanning industry.

In a DAF unit, both flotation and sedimentation of the particles and flocs generated in the coagulation-flocculation process is carried out. The clarified water is pressurized and saturated with dissolved air, when this saturated current enters the flotation chamber, depressurization occurs, generating micro-bubbles of air that drag particles and flocs that do not have enough weight to settle or sufficient buoyancy to the surface. Particles that naturally have sufficient sedimentability accumulate at the bottom of the DAF unit.

These equipments allow the extraction of floated and settled sludge with a very high solids content (up to 5%) and obtain a high-quality clarified water.

The sludge generated in a DAF equipment is directed to a dehydration treatment for which there are various techniques: centrifugation, pressure filtration, thermal drying or vacuum filtration.

There are also settling units that allow the separation of settleable solids, these equipments do not separate solids that cannot settle and they occupy more space than DAF clarifiers.

Alternative pre-treatments: there are other technologies capable of achieving considerable yields in the removal of chromium, sulphides and solids, however, they are less applied technologies due to their high cost: ion exchange, adsorption on activated carbon...

Combined application of coagulation-flocculation and DAF: technologies offered by SIGMA

This sequence allows the separation of contaminants by coagulation-flocculation and the removal of flocs and buoyant solids by applying a DAF system.

The flocs formed in the coagulation-flocculation process have an ideal size to be separated from the water in a SIGMA DAF dissolved air flotation unit. The DAF technology developed by SIGMA combines the principles of dissolved air flotation and sedimentation with optimal equipment design.

SIGMA offers different designs for the coagulation-flocculation process: stirred tank process or SIGMA PFL continuous process.

SIGMA DAF technology is an efficient and robust separation process for oils, fats, colloids, ions, macromolecules, microorganisms and fibers.

The sequence of coagulation-flocculation followed by DAF is a very common and widely used concept in the treatment of wastewater from the tanning industry and has proven over the years to be efficient in performance and costs, both operational and consumption of chemicals and energy when properly designed. It is an effective and robust pre-treatment for industrial wastewater treatment in general.

During DAF treatment, compressed air is introduced into a recycled clarification stream, dissolved in the liquid medium, and subsequently generates bubbles of 30 to 50 µm when released through a dispersion head into the flotation chamber. The coagulated and flocculated particles adhere to the bubbles and float to the top of the DAF unit, where they are removed mechanically.

The settleable matter descends into the sediment compartment at the bottom of the DAF unit and is discharged by a sludge extraction system.

SIGMA also offers equipment for the treatment of collected sludge.

The clarified water leaves the DAF unit via an adjustable supernatant-pumping system. Part of this stream of clarified water will be redirected by the recirculation pump to enter the compression and air saturation system.

SIGMA DAF technology is currently used effectively in other application areas such as drinking water treatment, tertiary wastewater treatment, sludge thickening, filter backwash waste recovery and seawater pre-treatment for desalination.

SIGMA offers a wide range of DAF equipment, specially designed according to the flow to be treated and space requirements, from units that can treat flow rates of 5 m3/h to equipment that can treat 1000 m3/h, compact equipment are also offered. The treatment capacity of SIGMA DAF equipment covers pollutant load ranges of up to 40 kg of solids per flotation surface.

We have a wide range of high-performance equipment suitable for different flow rates, each equipment is specially designed depending on the flow to be treated and its characteristics:

  • SIGMA DAF FPAC: for flow rates between 5 and 160 m3/h with a very high solid load. It is a high-performance cross flow system.

Download data sheet

 

Render of the DAF-FAPC equipment for the treatment of water flows between 5 and 160 m3/h from low to medium solid loads.

Figure 1. SIGMA DAF FPAC.

  • SIGMA DAF FPBC: for flow rates between 10 and 250 m3/h of low to medium solid loads. The equipment applies countercurrent flow and presents high performance.

Download data sheet

Render of the DAF-FPBC equipment for the treatment of water flows between 10 and 250 m3/h from low to medium solid loads.

Figure 2. SIGMA DAF FPBC.

  • SIGMA DAF FPHF: for flow rates between 200 and 1000 m3/h and high content of suspended solids. A combination of counter flow and cross flow is used for optimal performance.

Download data sheet

Render of the DAF-FPHF for the treatment of industrial wastewater with flow rates between 200 and 1000 m3/h with a high content of suspended solids.

Figure 3. SIGMA DAF FPHF.

  • COMPACT DAF: compact equipment installed in skids, in which the coagulation-flocculation system and the DAF are unified, allowing space savings and energy consumption.

Download data sheet

Render of the DAF- FPAC Compact for industrial wastewater treatment, compact design with high performance.

Figure 4. SIGMA COMPACT DAF.

DAF technology, accompanied by properly designed coagulation-flocculation chemistry, has been shown to be effective in removing grease, oils, suspended solids, turbidity, color, some bacteria, organic matter, recalcitrant organic matter, heavy metals, and other contaminants.

The advantages of SIGMA DAF systems include:

  • High quality of the treated effluent.
  • Quick start-up.
  • High-rate operation.
  • Thicker sludge (reduced sludge volumen production).
  • Reduced footprint.
  • Easy to operate.
  • Well-known and robust technology.
  • Simple, adaptable and efficient control systems.

The best combination of chemicals for the coagulation-flocculation stages and the corresponding pH adjustment must be designed based on the analysis of the waste water, applying Jar-Test tests to the samples of each waste water.

SIGMA offers to carry out this analysis and design the coagulation-flocculation and pH adjustment process and the DAF equipment, its construction, installation and commissioning. In our SIGMA LAB we can perform Jar-Tests to analyze a wide range of wastewater.

If you want more information regarding our services, do not hesitate to contact us through the form shown on the right, by writing us an email or by calling us.

3.2. Biological treatment

Currently most plants apply a BIOLOGICAL TREATMENT after homogenization and physical-chemical pre-treatment. Biological treatment, once inhibitory and toxic agents have been eliminated, is highly efficient since almost all of the organic matter present in wastewater is biodegradable.

The wastewater from tanning processes presents organic matter that is very easily biodegradable, therefore the biological treatment will be highly effective as long as the pre-treatment has been adequately designed for the elimination of the inhibiting agents described above.

Aerobic or anaerobic treatments can be used.

Aerobic treatment: The most widely applied is a simple system of activated sludge, this system can consist of a sequence of anoxic reactors (absence of oxygen) and aerobic reactors (introduction of oxygen) to eliminate COD and nitrogen through biological reactions carried out by microorganisms. This treatment can be designed as a continuous system or sequentially as an SBR ('sequencing batch reactor') system.

Figure 5. Biological treatment of active sludge in continuous by means of a combination of anoxic-aerobic reactors for the elimination of COD and nitrogen. Adapted from Artiga 2005.

Figure 5. Biological treatment of active sludge in continuous by means of a combination of anoxic-aerobic reactors for the elimination of COD and nitrogen. Adapted from Artiga 2005.

Figure 5. Biological treatment of active sludge in continuous by means of a combination of anoxic-aerobic reactors for the elimination of COD and nitrogen. Adapted from Artiga 2005.

Figure 5. Biological treatment of active sludge in continuous by means of a combination of anoxic-aerobic reactors for the elimination of COD and nitrogen. Adapted from Artiga 2005.

Anaerobic treatment: anaerobic systems can opérate with very high concentrations of COD and BOD5 and also allow the generation of biogas, a gas with a high energy value resulting from the biological process of elimination of biodegradable organic matter in the absence of oxygen.

The reactors most commonly used in tanning wastewater treatment are UASB (Upflow anaerobic sludge blanket). These reactors are compact, high-performance equipment where, in the same unit, COD removal, biogas collection and solids separation are carried out.

The main disadvantage of anaerobic systems is their high sensitivity to toxic substances and that the reduction of COD reaches usually no more than 60%, so it requires a well-designed pre-treatment and a subsequent aerobic biological treatment.

Diagram of the anaerobic upflow and sludge bed reactors

Figure 7. UASB reactors. Adapted from Bernardino S. 2019.

The following table shows the general elimination percentages resulting from the application of different series of technologies for the treatment of wastewater from the tanning industry:

Table 3. Estimated elimination of wastewater parameters from the tanning industry according to the treatment applied. Adapted from European IPPC Bureau.

 

COD

BOD5

SS

Chromium

Sulphide

TKN

Pre-treatment

Oils nad fat flotation

20-40

-

-

-

-

-

Sulphide oxidation

10

-

-

-

90

-

Chromium precipitation

-

-

-

90-99

-

-

Pre-treatment combinations

Homogeneization + sedimentation

25-35

25-35

50-70

20-30

-

25-35

Homog. + coag.-flocc. + sedimentation

50-65

50-65

80-90

20-50

2-10

40-50

Homog. + coag.-flocc. + flotation

55-75

55-75

80-95

20-50

2-5

40-50

Biological treatment (pre-treatment + …)

… aerobic

85-95

90-97

90-98

-

-

50

… anoxic – aerobic

85-95

90-97

90-98

-

-

80-90

… anaerobic UASB

65-75

60-70

50-80

-

-

20-30

3.3. Sludge management and treatment

The most suitable way to treat sludge depends on the availability of space, uses to which the sludge is intended for, etc. The sludge from the treatment of watewater from the tanning industry contains between 60-70% organic matter and 3-5% nitrogen, its potassium content is negligible.

Before treatment, the sludge must be dehydrated to facilitate its transport by reducing its volume. Equipment such as:

  • Band filters
  • Filter press
  • Centrifugal decanters

Where salts and/or polyelectrolytes may be applied to condition the sludge.

Several processes are used for the final treatment of sludge:

  • Anaerobic digestion: it also allows to obtaining biogas together with the digested sludge.
  • Application in agriculture: its direct application is allowed as long as the legal restrictions on the content of pesticides, pathogens, heavy metals and other pollutants are met.
  • Aerobic composting: generation of organic compost, it must also meet legal requirements.
  • Thermal treatment: incineration, gasification or pyrolysis can be applied. These methods allow energy recovery.

3.4. Advanced treatments with membranes

Typical pre-treatment and biological treatment processes are effective in removing sulfides, chromium, organic load, nitrogen and suspended solids, but the effluent still contains a large amount of salts and dissolved solids (sodium Na+, chlorine Cl-, sulphate SO42-, calcium Ca2+, magnesium Mg2+) and recalcitrant compounds.

Treated water cannot be reused if these contaminants are not removed. In industries with very high-water consumption, it is increasingly necessary to implement a CIRCULAR MODEL that allows the reuse of wastewater treated in the process. The technologies that allow the elimination of these pollutants and the reuse of water are MEMBRANES.

Membrane technology can be included in the treatment of wastewater from the tanning industry from two different points of view:

a) applying filtration membranes as post-treatment.

b) applying MBR technology (membrane bio-reactor)

3.4.1. Filtration membranes as post-treatment

There are four types of membranes in order from largest to smallest pore size: Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF) and Reverse Osmosis (RO). In Figure 8 this classification is represented according to the pore size (in nm) and the compounds that each type is capable of retaining.

Diagram of the classification of membranes applied in advanced industrial wastewater treatment.

Figure 8. Classification of membranes applied in wastewater treatment.

Microfiltration MF: MF membranes remove colloids, suspended solids, bacteria and viruses. They are generally applied as a pre-treatment for other UF, NF or RO membranes. They are not applied alone since their effectiveness is poor for the effluent requirements.

Ultrafiltration UF: UF membranes consist of a selective filtration process by applying pressure of up to approximately 10 bars. These membranes eliminate high molecular weight organic macromolecules and particles, allowing the fractionation of organic matter. The effectiveness of a UF membrane is strongly dependent on the type of material the membrane is made of.

UF membranes are generally applied as a pre-filtration to optimize RO processes and prevent clogging. UF membranes are the most commonly installed in MBR systems.

Nanofiltration NF: NF membranes are applied for the removal of recalcitrant organic matter and heavy metals in wastewater treatment. These membranes generate very little volume of concentrate. NF membranes require careful control for clogging.

Reverse Osmosis RO: RO is a highly efficient pressure-applying technique for wastewater purification. An RO process allows all dissolved solids, traces of organic compounds, heavy metals and monovalent ions to be concentrated.

In the treatment of wastewater from the tanning industry, RO membranes in combination with UF or NF achieve removal efficiencies of around 95% of dissolved solids, 94% of Na+ and Cl- ions, 98% of sulphates, 65% of Mg2+ and 55% Ca2+. The main disadvantages of an RO system are its great tendency to clogging (so a previous treatment is necessary, such as the installation of UF membranes) and the generation of a large volume of concentrate (which has to be treated appropriately).

The biggest challenge today in the application of membrane technologies in wastewater treatment is to achieve a cost-efficiency balance and to correct clogging problems. The selection of cleaning products and methods is very important when designing and installing a membrane system to optimize both operational cost and performance.

3.4.2. Membrane bio-reactors MBR

This technology consists of the application of ultrafiltration membranes within the biological treatment as a means of retention of biomass.

The membranes act as a separator for the sludge generated in the reactor, which implies that the installation of a subsequent clarifier is not necessary, it also prevents the loss of biomass (loss of nitrifying microorganisms and microorganisms capable of degrading slowly biodegradable organic matter), achieving very high biomass concentrations inside the reactor.

These reactors are equipment with a very high biomass concentration, very low sludge production and high pollutant removal performance, in addition, they allow a flexible design and operation. After an MBR reactor, it is very common to install RO reverse osmosis membrane units as post-treatment.

The main disadvantage of these systems is the fouling of the membranes, which is why a carefully designed cleaning system is necessary.

diagram of MBR configurations: per external membrane module and per internal or submerged membrane module. (Artiga 2005).

Figure 9. MBR configurations: a) external membrane module and b) submerged or internal membrane module. Adapted from Artiga 2005.

Membrane technologies offered by SIGMA.

SIGMA offers its membrane technology for the design and installation of Ultrafiltration, Reverse Osmosis and MBR reactors.

Photo of the MBR (Membrane Biological Reactor) membranes installed by SIGMA for the biological treatment of industrial wastewater.

Figure 10. Membranes for MBR installed by SIGMA.

SIGMA DAF SMBR reactors offer the following advantages:

  • PLUG&LAY solution.
  • They offer maximal reliability and durability.
  • They allow for a constant quality of the effluent.
  • Compact plant that allows modular addition of UF membranes.
  • Its operation and control are simple and robust.
  • They have high resistance to oxidizing agents.

SIGMA DAF SMBR plants are specifically designed for each wastewater to be treated. The range of capacities is between 20 - 100 m3/day.

Render of the SIGMA SMBR unit with a compact design for the biological treatment of industrial wastewater, manufactured by SIGMADAF Clarifiers.

Figure 11. Compact units SIGMA SMBR.

4. SIGMA case study in tannery wastewater treament

Below is a successful case of the installation of a wastewater pre-treatment plant for the tanning industry, designed, manufactured and installed by SIGMA.

Shoe tanning

Wastewater origin

Mixed wastewater from the shoe tanning process

Objective

Compliance with the legal limits of discharge to the sewage network, which will be addressed to an WWTP and the possibility of projecting a reuse of the effluent. Complete sludge treatment.

Flowrate

200 m3/day

Wastewtare characteristics

· pH = 3

· TSS = 1000 mg/L

· COD = 3000 mg/L

· BOD5 = 1500 mg/L

· Total nitrogen = 100 mg/L

· Cr3+ = 140 mg/L

Equipos instalados

· Sieves

· Homogeneization tank

· Coagulant (FeCl3), flocculant (polyelectrolyte) and pH control (NaOH) preparation and dosing system

Coagulation, flocculation and pH control reaction tanks system

· Clarifier SIGMA DAF FPAC-40

· Full sludge treatment system: conditioning and dewatering by filter press, preparation and dosage of milk of lime (Ca(OH)2)

Performance

> 90% TSS removal

> 67% COd removal

> 67% BOD5 removal

> 50% Total Nitrogen removal

> 98% chromium Cr3+ removal

Sludge dry matter content = 35%

Photographs of SIGMA installations in the treatment of wastewater from tanning industries:

Clarifier SIGMA DAF FPAC-40:

Side photo of a SIGMA DAF-FPAC clarifier installed in a wastewater treatment plant of a tanning industry.

Aerial photo of a SIGMA DAF-FPAC 40 clarifier installed in a wastewater treatment plant of a tanning industry.

Filter press:

photo of the filter press equipment for the dewatering of sludge from the treatment of wastewater from the tanning industry installed by SIGMA.

Coagulant, flocculant and NaOH preparation and dosing tanks:

Panoramic photo of the coagulant, flocculant and soda preparation and dosing tanks for the treatment of wastewater in tanneries.

5. References

Álvarez S.G., Maldonado M., Gerth A., Kuschk P. Caracterización de Agua Residual de Curtiduría y Estudio del Lirio Acuático en la Recuperación de Cromo. Revistas Cite.

Artiga P. 2005. Contribución a la mejora del tratamiento biológico de aguas residuales de la industria de curtidos. Memoria Tesis Doctoral Universidad de Santiago de Compostela, Departamento de Ingeniería Química.

Bernardino S. 2019. Production of biogas/bioSNG from anaerobic pretreatment of milk-processing wastewater. Chapter in ‘Substitute Natural Gas from Waste. 397 – 424.

Córdova H.M., Vargas R., Cesare M.F., Flores L., Visitación L. 2014. Tratamiento de las aguas residuales del proceso de curtido tradicional y alternativo que utiliza acomplejantes de cromo. Revista de la Sociedad Química del Perú. 80(3), 183-191.

Estudio de Minimización. Sector: Curtidos. Ministerio de Obras Públicas, Transportes y Medio Ambiente. Subdirección General de Resíduos.

European IPPC Bureau.

Song Z., Williams C.J., Edyvean R.G.J. 2003. Treatment of tannery wastewater by chemical coagulation. Desalination. 164, 249-259.

Suárez A.F., Agudelo R.N. 2014. Tratamiento de agua residual procedente de la industria de curtiembres mediante humedales subsuperficiales usando Zantadeschia Aethiopica. Avances Investigación en Ingeniería. 11(1), 121-126.

Suthanthararajan R., Ravindranath E., Chitra K., Umamaheswari B., Ramesh T., Rajamani S. 2003. Membrane application for recovery and reuse of water from treated tannery wastewater. Desalination. 164, 151-156.