The advantages of Zero Liquid Discharge (ZLD) systems have been examined and demonstrated several times in national and international projects. However, initially, there was either a lack of an immediate need or the necessary framework conditions for the economic implementation of a ZLD system. By applying the Remondis Aquatic Mining® approach in India, Remondis Aqua has reorganised the wastewater treatment at a chemical plant by designing and implementing a ZLD plant and subsequently operating and maintaining it as the service provider on a BOOT Model (Fig. 1). The decisive factors for the implementation of this ZLD plant were the periodic prevailing water shortages in the region and limited wastewater infrastructure with high local quality requirements. By implementing a ZLD approach, the customer is now supported with more flexible options for adjusting and expanding its production capacities and benefits from the recovery of significant quantities of high-quality raw material in the form of salt.
Commissioned at the beginning of 2022, the ZLD plant has continuously supplied treated water to meet the high-quality production requirements required at the chemical plant.
The ZLD plant treats variously composed influent streams individually and combined over several stages with various methodologies. The wastewater consists of production and cleaning water and is fed to the ZLD plant in a batch process. The wastewater streams from the production of catalyst products contain heavy metals and have a high sodium and sulphate content. In addition, the domestic wastewater from the production site is also treated. A total of up to 600 m3 of wastewater is treated daily. (refer Table 1).
Wastewater produced at the chemical plant is separated during production into partial streams according to its composition. The main influent streams are divided into "low TDS" wastewater (total dissolved solids), "high TDS" wastewater, "non-recovery" streams and sewage/ canteen wastewater. The characteristics of the main influent streams are shown in Table 1.
Table 1: Wastewater characteristics of main inlet flows of ZLD plant
Wastewater treatment is carried out using multiple process stages and treatment routes due to the different compositions of the wastewater streams. An overview of this process is provided in Figure 2. Permeate from the water recycling system and condensate from the two evaporator lines provide recovered service and production water which is used by the customer in the chemical plant and for internal water demands of the ZLD plant. Recovery of recyclable material in the form of very high-quality salt creates a commercial grade product that brings with it a significant economic benefit. The minimisation of disposal quantities is made possible by a combined treatment of internal streams. In the following paragraphs, the treatment lines are described in detail and characteristic values are shown.
Water Recycling System – Low TDS Wastewater
The first treatment line for the treatment of the low TDS wastewater uses conventional wastewater treatment processes which are coordinated to ensure maximum cleaning performance and corresponding operational reliability.
In the first step, appropriately dimensioned equalisation tanks are used simultaneously to balance the inflow quantities and the wastewater composition. The pH values are then adjusted for the subsequent precipitation. Due to the high concentrations of heavy metals, especially nickel and aluminium, precipitation and flocculation provide important pre-treatment steps in the system. The solids are separated by lamella separators with subsequent sludge treatment and disposal. Via filtration stages (multi-layer and activated carbon filters), with downstream ion exchangers, the wastewater is fed to an ultrafiltration stage for final treatment in a two-stage reverse osmosis which can achieve a yield of up to 85 %. The permeate from both reverse osmosis stages is transferred directly to the customer's storage tanks for production water. During operation, internal wastewater streams, such as backwash in the area of the filter and ultrafiltration unit or wastewater from sludge treatment, are fed directly back into the system. The concentrate from the second stage of reverse osmosis and the wastewater from regular cleaning and regeneration in the ion exchange, ultrafiltration and reverse osmosis units are fed back into the system.
Table 2: Generated process water quality of the ZLD plant
Multi-effect Evaporator Unit 1 – High TDS Wastewater
In addition to wastewater treatment and production of service and production water, salt (sodium sulphate) is recovered in significant quantities in the ZLD plant. This product is sold on the raw materials market due to its high quality, as a purity of over 99% is achieved during recovery. A necessary condition for achieving this high quality is the separate pre-treatment of the High TDS wastewater in the second treatment line to reduce contamination in the salt. For this purpose, the wastewater volume in the equalisation tank is regulated and the pH value is adjusted by dosing acid and alkali. Analogous to the low TDS wastewater, mainly nickel and aluminium are precipitated. Flocs get separated in a sedimentation tank after the addition and mixing of flocculation aids.
Carbon dioxide (CO2) is then stripped in a cascade of basins at low pH values (pH < 4) by means of aeration. The final stage of pre-treatment involves neutralisation of the pre-treated wastewater for transfer to the evaporator unit (Multi Effect Evaporator - MEE). The condensate collected in the three-stage evaporator unit is reused as service and production water. On the other hand, the water content can be reduced via the evaporator stages with subsequent crystallisation, solids separation and drying to such an extent that the salt can be filled directly as a saleable bag product (Table 3). The saline slurry (mother liquor) reaches a saturation in the system of 30%. The purge in the crystalliser is run in concentrations of 5-10 % and transferred to the treatment line for non-recyclable streams.
Table 3: Product quality (Sodium sulphate – Na2So4)
Multi-effect Evaporator Unit 2 – Non-recovery streams
In the third treatment line, various waste waters from production as well as internal waste waters from the ZLD plant are brought together. After treatment in the water recycling system, the wastewater is pre-treated by means of folding and flocculation as well as solids separation in the lamella separator. The condensate and the concentrated brine are separated in a two-stage evaporator unit. Due to the high level of contamination by heavy metals and other impurities, this stage is exclusively performed with a drum dryer to enable subsequent disposal of the contaminated salt.
Remondis Aqua was able to plan and construct a ZLD plant on behalf of the customer that is optimally adapted to the wastewater characteristics and enables efficient recovery of service and production water as well as a high-quality product during operation. Problems due to a deviating feed matrix to the plant can be minimised through years of planning and operating experience or mitigated through the possibility of modular expansion. The operation of the ZLD plant is currently being economically and technologically optimised on a daily basis and stabilised operationally to enable efficient and economical recovery of resources for the customer.