When it comes to industrial wastewater, extraordinary problems need extraordinary solutions

After a major petrochemical company in Saudi Arabia completed its main product process design and development, they realized that a very important issue was left unattended, namely, the plant’s difficult wastewater stream.

The 9,000 kg/ hr waste was predominantly 4.5% sodium sulfate, along with other chemical COD/ BOD in various concentrations, as shown in Table (A).

Table A

The wastewater presented a challenge to the client, as it was something of more than expected. The discharge limits were specified as shown in table (B).

Table B

An initial proposal set by the client suggested using Reverse Osmosis as a possible treatment technology for the wastewater. The concentration levels indicated above, however, rendered this solution unviable due to the following reasons:

a) The high inlet COD, solvents, and organic chemicals would foul, damage, and/ or dissolve the RO membranes in very short period of time, and
b) The RO will yield a concentrated stream of these chemical thus exceeding the client’s set criteria of no liquid waste at the end of the treatment.

AES engineers started working on an array of alternative methods to handle this waste stream. A direct biological treatment for the incoming waste was not feasible due to the elevated feed salinity. This could hinder the biological growth of activated sludge and result in insufficient treatment. With this in mind and the exclusion of the RO technology, one valid option offered a way out, desalination by thermal evaporation.

A two-stage evaporation process was selected where the wastewater is initially introduced to a falling film evaporator (FFE) followed by a vacuum cooling evaporative crystallizer (VCEC). The FFE is run under atmospheric pressure giving more reliable operation and minimizing the risk of failures due to leaks when run under vacuum.

A Single stage mechanical vapor recompression fan was used to generate the required energy needed to pressurize the steam vapor thus heating it up and causing more vapor to form. The slurry from the FFE, now contains around 20% dry solids content is pumped under high temperature to the VCEC crystallizer. In the VCEC, vacuum is maintained in the sealed flash evaporation tank. The sudden change in pressure causes more water to evaporate from the slurry, raising the concentration and forming crystals of sodium sulfate salts. The vapor flashed in the VCEC is condensed and sent along with condensate from the FFE to the biological treatment.

A progressive cavity pump moves the slurry (containing tiny crystals) from the VCEC tank to a specially constructed pusher centrifuge. The centrifuge raises the solids content to more than 90%, the crystals chutes down to a fluidized bed drier where it dries to virtually no moisture content. The dried sodium sulfate crystals are then pneumatically conveyed to two powder storage silos where they are stored waiting to be emptied into the next dry powder bulk transfer truck. The sodium sulfate crystals have now a commercial value and can be sold to other industries like glass manufacturing offsetting some of the running cost of the treatment plant.

An ejector is connected to the FFE vents; with condensate as a motive liquid, slight vacuum is created in the vents in order to withdraw any escaping gasses and non-condensables. The mixture is pumped to the biological treatment section preventing possible ambient air emissions if fugitive gases were to be vented to atmosphere.

The condensate collected from the evaporation steps is pumped through a plate and frame heat exchanger heating up the cooler incoming wastewater and recovering some of the heat (otherwise lost) and distributed to four long-retention biological reactor tanks.
High concentration of activated sludge (MLSS) is maintained in the biological reactor tanks (range of 10,000 to 15,000 ppm.) This high concentration coupled with the long retention time helps breaking down the refractory COD and reducing the toxicity through dilution with the bulk volume of water in the bioreactor tanks.

Due to high levels of MLSS, two subsequent membrane bioreactor tanks (MBR) are used to extract treated water from the biological system which can either be re-used in the plant or disposed-off safely. Activated sludge is collected and disposed-off as non-hazardous waste.
The plant has state-of art design features and offered a cost-effective and reliable total solution to a stubborn wastewater problem while meeting environmental regulations and the clients set discharge criteria.

Asad Iqbal Khan | Senior Executive - International Business | AES Arabia Ltd.
Phone: +966-1-4772398 Ext. 1180 | Fax: +966-1-4785456 | Mobile: +966-555-207569 | www.aesarabia.com