Stem the flow

14 June 2019

Jakov Buljan, KV Emmanuel, M Viswanathan, M Bosnic and I Král present analysis of flow and energy aspects of zero liquid discharge (ZLD) technology in the treatment of tannery effluents in Tamil Nadu, India.

In the course of conventional treatment of tannery effluent, the composition of total dissolved solids (TDS) somewhat changes, but its total level remains virtually constant and considerably exceeds typical discharge norms. The global trend of processing of fresh hides – salt-free raw material – is continuously expanding; for a host of reasons, this approach is non-existent in India.

The precarious situation with water and soil pollution in the area of tannery clusters along the Palar River prompted the state environmental authorities to press for adherence to TDS discharge limits as well as to impose an approach not practiced in the tanning industry: a zero liquid discharge (ZLD) concept. Essentially, the ZLD systems concentrate dissolved solids by reverse osmosis (RO) and some kind of multi-effect evaporation (MEE) until only damp solid waste remains. Solid waste is disposed and nearly all water is reclaimed and reused.

Accordingly, some of the existing common effluent treatment plants (CETPs) have been supplemented by RO and MEE, together with auxiliary steps (tertiary treatment or water softening). The analysis investigates and relates raw and equalised effluent inflows, RO feed, permeate and reject, evaporator feed and condensate, and the yield of recovered, reusable water. Since the energy costs are critical for the viability of the entire concept, data about energy consumption (thermal, electrical main and diesel) at key stages (RO, multistage evaporation) are consolidated, analysed and correlated. Additional energy needs and costs are compared with those for conventional (CETP) treatment and estimates made of the carbon footprint increase caused by the ZLD operations.

Address environmental concerns

In the tanning area of Ranipet, Vellore District, there are about 200 tanneries grouped in three clusters, each serviced by a CETP, with treated effluent ultimately ending in the Palar River, which in the recent years is mostly dry with no flow. There are neither sewerage networks nor sewage treatment plants in the adjacent municipalities.

TDS, mainly chlorides and sulphates in tannery effluent, have become the major environmental concern in arid and semi-arid regions as they make the receiving water recipients unfit for livestock watering and irrigation. Although a certain percentage emanates from pickling, deliming, tanning and wet finishing, the main source of TDS, especially of sodium chloride, is salt from preservation. It is estimated that worldwide at least three million tonnes of common salt a year is discharged into water recipients and specific discharge limits for TDS vary.

Environmental damage caused by salting gradually prevails over its convenience aspects; the tanning industry in Europe has already largely switched to processing of salt-free raw material and this trend is continuously expanding.

For a host of reasons, while enforcing the TDS limit of 2,100mg/L, state environmental authorities and the tanning industry have chosen a different strategy: to adopt a ZLD approach. The existing CETPs, following the usual treatment technology, have been supplemented by advanced, energy intensive methods like RO and MEE, together with the necessary auxiliary steps (again, tertiary treatment or water softening, for example).

This paper attempts to analyse effluent flows, energy aspects and the impact on carbon footprint of the ZLD segment at three CETP+ZLD systems in Vellore District after a few years of operations. In that context, experience from the CLRI – UNIDO project in 1998–2000 in operating a pilot two-stage RO plant of 1m3/h capacity (albeit using solar pans instead of advanced evaporators) proved quite useful. The conclusion was that the system per se was technically viable but that O&M costs (only partly offset by the price paid for fresh water) were quite prohibitive, mainly due to high energy inputs.

Selection of plants for analysis

The plants selected cover the three main types of clusters – processing raw hides/skins to finished leather, (RANITEC), predominantly from raw to wet-blue (VISHTEC), and from wet-blue/EI to finished leather (SIDCO). The three plants basically follow the same technology, are operated by quite professional staff and the managements are willing to cooperate. They are all connected to the Care AIR centre (server) of the Tamil Nadu Pollution Control Board (TNPCB), the flow data is recorded in real time and counterchecks are possible.

Water consumption, effluent flows, yield

One claim is that the addition of the ZLD stage has resulted in water consumption decrease from about 28L/kg to only 11–12L/kg of wet-salted weight; increase in concentrations of pollutants support that claim. The opposing view is that local tanners already have long experience in economising with water brought by tanks from considerable distances – to further halve such low consumption within three to five years does not look likely. In addition, according to some UNIDO studies, the theoretical minimum is about 12L/kg and requires sophisticated recycling equipment.

The permeate from the RO system and the condensate from the evaporator are combined and distributed back to the tanneries through a recovered water conveyance system. There are perplexing figures and proportions:
¦ RO feed versus inflow to CETP ratio varies from 81 to 114, average 99%.
¦ Permeate versus RO feed varies from 57 to 80, average 72%.
¦ RO reject versus RO feed varies from 20 to 43, average 28%.
¦ Evaporator condensate versus feed varies from 97 to 109, average 103%.
¦ Total recovered water versus RO feed from 96 to 106, average 102%.
¦ Total recovered water versus inflow to CETP varies from 80 to 113, average 101%.
¦ Salt produced is 5,043t, from 9.7 to 14.3, average 12.1kg/m3.

For an accurate flow balance, it would be necessary to take into account additions such as water used for dissolving of chemicals and water from boilers, as well as all losses (evaporation, sludge).

The overall flow balance is from the tanner’s viewpoint satisfactory; all losses due to evaporation (rather low due to high air humidity) and water removed with sludge are compensated by additions for dissolution of chemicals, water softening and washes. Ultimately, the effluent inflow coincides with the volume of water sent back to tanneries for reuse; its quality is superior to fresh water, due to low hardness.

“One claim is that the addition of the ZLD stage has resulted in water consumption decrease from about 28L/kg to only 11–12L/kg of wet-salted weight.”

The ZLD is not so much treatment but rather a salt removal and sequestration system. For good results, it should operate with constant chemistry and flow – for which they were specifically designed – and must be monitored continuously. Preparatory, post-CETP ‘conditioning’ steps (in particular water softening) often require dosing of different chemicals, including salts, which is quite a paradox for what is essentially a salt removal system. The permeate from the RO system and the condensate from the evaporator are combined and the recovered water is distributed back to one-day storage capacity tanks in individual tanneries. The salt-laden solid residue is stored in bags in a huge salt storage yard.

Norms, monitoring

Water used in tanneries in clusters in the Vellore District is in most cases a mixture of water from own drilled wells and (better) water drawn from the Palar River bed further upstream and brought by tankers; the supply and characteristics of fresh water are inconsistent and unpredictable, and comprehensive analyses of fresh water are apparently not available. Reportedly, the TDS of fresh water is in the range of 800–1,500mg/L, hardness 200–800mg/L (tankers) and 1,000–3,000mg/L, hardness 800–2000mg/L (own wells). Thus, the usual problem of TDS is compounded by the high TDS/hardness level of fresh water. To meet the TNPCB discharge norms for dissolved solids (inorganic), 2,100mg/L, chloride 1,000mg/L and sulphates 1,000mg/L, a very different set-up in the whole supply chain – mixing of treated effluent with municipal waste water and/or advanced methods of decreasing the TDS level – is required. Unfortunately, differences in values found by CETP’s own laboratories and analyses carried out by independent laboratories (third parties) too often exceed normal and acceptable variations. Inevitably, this casts a kind of shadow of doubt and possibly undue reserve in considering the laboratory statistical data. The computerised operations management system for the Ranipet CETP includes analytical data for key treatment units as well as sludge disposal records, and sludge and leachate analysis.

Energy considerations

Energy consumption in tanneries depends on factors such as tannery location (geographic zone), production method, equipment, performance of electric motors, the ratio of manual versus mechanical/automated handling (for example, in moving the hides), drying methods, solid waste treatment or effluent treatment technology.

Generally, water (float) heating and drying, almost equally, constitute about two thirds of the energy consumption for leather processing itself. The type of energy source is also very relevant – fossil fuel (natural gas, coal, diesel), renewable (wood, biomass) or self-generated renewable (solar energy, wind). Optimisation of electric motors, use of electric motors with higher efficiency and reducing the level of reactive energy are an important part of (electric) energy savings measures. The use of diesel generators is limited to emergencies.

The impact of addition of the ZLD stage (RO+MEE) to the conventional treatment can be summarised as follows:

¦ The consumption of electrical energy went up nearly three times.

¦ The overall energy consumption (electrical and thermal) went up nearly 15 times.

¦ The cost of electrical energy, including its unit cost (Rs/m3 ) went up nearly three times.

¦ The total cost of energy (electrical and thermal) went up about 4.5 times.

¦ The share of ZLD energy in total energy consumed is about 94%.

¦ The share of ZLD energy cost in total energy cost is about 78%.

Chemicals from the ZLD stage, O&M costs, salt residue

In addition to the usual chemicals used during the primary treatment (lime, alum, polyelectrolytes) significant amounts of chemicals affecting the TDS content are added during tertiary treatment, water softening, RO and evaporation steps, such as hydrochloric acid, sodium metabisulphite, antiscalant, polyphosphates, caustic soda or sodium bicarbonate.

The reported, indicative O&M cost for the year 2015/16 is between $6.9–8.7/m3, assuming part of it is offset by saving the cost of fresh water (about $1.40/m3). In the absence of reliable data about raw material input or yields, it is not possible to relate the O&M cost to leather output; educated guesses put them from about Rs20/m2 (RANITEC), Rs23/m2 (SIDCO) to Rs40/m2 (VISHTEC), corresponding to $0.30, $0.35 and $0.60/m2.

The salt residue represents a very serious environmental challenge; the quantities generated are impressive. In 2015/2016, the RANITEC plant produced 5,043t, VISHTEC 1,818t and SIDCO 1,591t. Unfortunately, currently there are substantial differences between the theoretical values for the RO plus evaporation stage and the actual outputs of salt residue at the three plants considered.

There are views and computations suggesting substantially lower figures. According to them, the unaccounted loss at RANITEC is 4.65%, at SIDCO 3.72%, and only 0.15% at VISHTEC. However, some logic and estimates in those computations – such as the share of the volatile portion of salt lost in evaporation, or in transportation and some others – are very questionable. Obviously, the complexity of the issue requires extensive, independent monitoring and analysis over at least one year.

Carbon footprint: the impact of ZLD stage on CO2 emissions

Values used for computations are as follows:

¦ Average CO2 emissions for electricity production in India: 0.9kg CO2/kWh (2012).

¦ Calorific value of diesel used by DG: 39MJ/L.

¦ CO2 emissions from diesel: 74.1kg CO2/GJ of thermal energy.

“It is quite late but possibly not too late to thoroughly (re)consider potential alternatives, a combination of short and long-term options.”

¦ CO2 emission per litre of diesel: (39×74.1)/1,000 = 2.9kg CO2/L of diesel.

¦ Calorific value of firewood used by evaporation boilers: 16.5MJ/kg.

¦ CO2 emissions from firewood burning: 109.6kg CO2/GJ of thermal energy.

¦ CO2 emission/kg of firewood: (16.5×109.6)/1,000 = 1.8kg CO2/kg of firewood.

¦ COD of effluent before biological treatment: 2,490mg O2/L.

¦ COD of effluent after secondary clarifier: 260mg O2/L.

¦ COD degraded during biological treatment: (2,490–260) = 2,230mg O2/L.

¦ Estimated COD:TOC ratio: 3:1.

¦ CO2:TOC ratio: 3.67:1.

Impact and potential of ZLD

The dramatic situation with water and soil pollution along the Palar River together with pressure from the public and buyers eventually prompted the TNPCB to enforce the discharge limit for dissolved solids (inorganic) of 2,100mg/L; apparently, the ZLD system was imposed as the only approach to supplement the conventional treatment.

Reportedly, this has resulted in water consumption close to the theoretical minimum (12m3/t) and substantial underutilisation of CETP and ZLD plants. A very strong opposing view is that first, the tanners from the area already had long experience in economising with water; second, a rather complex water-saving and float-recycling system is required to achieve such a low level; third, the necessary technology modifications take time; and fourth, close, independent scrutiny is needed to verify this claim.

The average yearly flow rates along the treatment line in three ZLD plants considered are the RO feed versus inflow of 99–101%, RO permeate versus RO feed 72–76%, RO reject versus RO feed 24–28% and the total recovered water versus inflow 97–102%.

The reported, indicative O&M cost for the year 2015/16 is $6.90–8.70/m3, assuming part of it is offset by saving the cost of fresh water (about $1.4/m3).

The salt residue produced poses a very serious environmental challenge; in 2015/16 it was 5,043t (RANITEC), 1,816t (VISHTEC) and 1,591t (SIDCO). Unfortunately, there are substantial differences between the theoretical values and the actual outputs; large quantities are ‘missing’ without a convincing explanation.

Computing average CO2 emissions for electricity production in India, calorific value of firewood used by evaporation boilers, CO2 emissions per kilogram of firewood, COD degraded during biological treatment, estimated COD/TOC ratio and CO2/TOC ratio, it works out that the ZLD stage has increased the CO2e (CO2 equivalent) emissions at RANITEC by more than six times.

There is no doubt that industrial scale ZLD in the treatment of tannery effluents is technically feasible, and recycling of the purified water both logical and practical. However, the system is not robust, and a viable solution for reutilisation and/or safe disposal of solid residue is not in sight; moreover, within about three years O&M costs may exceed the installation cost.

It is quite late but possibly not too late to thoroughly (re)consider potential alternatives, a combination of short and long-term options such as construction of proper sewage systems and waste water treatment works in the townships in the Vellore District; simultaneous strong support for organised slaughter of some livestock (buffaloes, goats/sheep) and salt-free preservation; or the concentration of wet-blueing works.

Finally, further work by a multidisciplinary ground team is needed to more closely study issues such as detailed water mass balance, the exact impact of chemicals added and changes in the TDS composition along the process, optimisation of auxiliary processes (ultrafiltration, water softening) and possibly establishing more rigorous data recording.

Industrial scale ZLD to treat tannery effluents is technically feasible, but reliability, residue disposal and cost issues remain.
ZLD concentrates dissolved solids by reverse osmosis and multi-effect evaporation until only damp solid waste remains.
Polution of the Palar River prompted the state environmental authorities to act.

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