Crude oil containing sulphur, often in the form of hydrogen sulphide, in excess of 1%, is referred to as sour crude oil. Refineries that process sour crude oil generate sour water in several ways.
The processes of desalting, fractional distillation in atmospheric or vacuum towers, cracking in thermal or catalytic units, and hydrodesulphurisation (HDS) all lead to a sour water by-product. Other significant sour water contaminants include ammonia, nitrogen, phenol and cyanide.
The first treatment stage of sour water is typically done in a sour water stripper, whereby large quantities of hydrogen sulphide and ammonia are steam-stripped from the water. However, the high contamination levels found in the remaining stripped sour water requires additional treatment prior to discharging into the environment, or recycling for other refinery processes.
Refinery wastewater treatment
At this stage, many refineries send the stripped sour water, together with other refinery wastewater streams, for further wastewater treatment, as illustrated below in Figure 1.
Typically, the combined effluent undergoes a gravitational separation (API). Due to density differences, free oil is separated from the water and forms an oily upper layer, which is then skimmed off. Heavy components such as sand or other particles are collected from the bottom of the tank, and the water flows to the next stage of separation. Dissolved Air Flotation (DAF) or Dissolved Gas Flotation (DGF) primarily allows separation of dispersed oil from the water.
The separation is achieved by forcing air into a solution within the wastewater through a high pressure system. The pressure is then released, causing the air to form tiny bubbles which adhere to the oil globules. This process elevates the oil globules which can be easily extracted by skimming or filtration. The pre-treated wastewater subsequently undergoes a biological treatment in which dissolved contaminants are biodegraded.
The most common biological method is known as activated sludge. The dissolved contaminants are consumed by the microorganisms within the tank to form additional bio-mass (sludge). Water and sludge are washed out of the bioreactor and into a clarifier. The solid waste (sludge) that settles on the bottom of the clarifier is composed of both live and dead bacteria. Approximately 50-70% of this sludge is transported back to the bioreactor (note the two-way arrow between the clarifier and the bioreactor in Figure 1), in order to continually re-activate the biological process.
The remaining sludge is fed into a de-watering system for removing additional water and is then disposed of in a landfill. The effluent from the clarifier passes through a filtration unit (usually an ultra-filter) into a settling pond and only then is ready for discharge or recycling.
Challenges of stripped sour water treatment
The composition of stripped sour water is not consistent and due to fluctuations in the types and levels of contamination, it often causes upsets and overloads during the treatment process. Attempts at mixing and diluting the stripped sour water with the main refinery effluent stream, prior to the primary treatment, have often been unsuccessful. Even after dilution, the contamination levels are often too high, causing a toxic effect on the microorganisms in the biological process, which lead to system failures.
An additional problem is that the typical biological process does not always reduce the contamination to the required discharge levels.
This is especially true when the initial nitrogen or phenol levels of the stripped sour water are high. In such cases the final nitrogen and phenol levels in the treated water might not meet discharge requirements, even after the complete treatment process. Even in cases where the activated sludge treatment succeeds in meeting the required discharge levels, the process yields a high sludge by-product that translates directly into higher disposal costs.
An alternative biological approach as a solution
A recently developed biological method for wastewater treatment, known as the Automated Chemostat TreatmentTM (ACT) appears to provide a potent solution for the existing challenges in this field. This new biological approach is based on maintaining a pre-selected bacterial ‘cocktail’ within a bioreactor at a stable, low concentration, while monitoring the system with a fully automated control unit.
The first essential aspect of this biological method lies in the meticulous selection and culturing of bacteria from pre-treated waters, which are then specifically designed for any given wastewater type. These bacteria are naturally occurring, without alteration or genetic engineering.
The increased homogeneity of the cocktail ensures a more targeted and effective bio-degradation of the polluted water content. The bacterial cocktail is so specifically designed for the water-type and on-site environmental conditions, that some designs even yield a cocktail tolerant to extreme environments, such as high temperatures (up to 45 ºC), or high salinity (up to 4%).
Additionally, as the bacteria concentration is kept to a minimum throughout the process, aggregate formation is prevented. This approach increases the surface area available for the bacterial bio-degradation process, resulting in a higher quality of effluence. Moreover, the low cell concentration ensures that a young cell population is maintained, keeping sludge buildup at a minimum, and cell efficiency at a maximum.
Lastly, the process is continuously monitored by a control system, designed to overcome system fluctuations. The monitoring occurs both in pre-treated influent as well as within the bioreactor. The fully automated control system maintains a permanent homeostatic state, and can handle a variety of contaminants and waste capacities.
This type of custom-designed solution effectively decreases the issues surrounding the treatment process. The sludge levels produced by the ACT™ solution are significantly lower in comparison to sludge levels produced by traditional treatments. This directly translates into a cost reduction, as less handling of by-products is required.
Implementation of ACT™
The modular approach provided by this solution allows for on-site implementation that leverages existing infrastructure. As such, when treating stripped sour water streams, the ACT™ system can be integrated in a complementary fashion with the refinery main stream process. With this option, stripped sour water passes through the API and DAF units and then enters the ACT™ bioreactor. Following the biological treatment, the treated stripped sour water can be returned to the main bioreactor for further biodegradation, filtration and discharge. Alternatively, it can be implemented as a completely separate process, solely dedicated to the stripped sour water stream. In this option, after undergoing API and DAF treatments, the stripped sour water can be treated in the ACT™ bioreactor, filtered, and then directly discharged to the environment, or recycled.
The complete separation of the stripped sour water from the main stream is advantageous in two regards. Firstly, such water, after proper treatment, can be recycled for additional usages in various refinery processes. Secondly, the total organic load in the main stream is significantly decreased, allowing for higher treatment capacity and efficiency.
The efficacy of the ACT solution with regard to the treatment of stripped sour water was recently demonstrated by BioPetroCleanTM (BPC), developer of ACT™.
A stripped sour water sample was treated using the BPC-ACTTM technology in a continuous mode. During the treatment process, the water was analysed for nitrogen, phenol, solid levels, COD and TOC reduction. The water in the reactor was kept at room temperature, and at a constant DO and pH. In addition to the high carbonaceous contaminants, the treated water contained high levels of nitrogen and phenol. During the BPC-ACTTM process, the carbon uptake was very high (as indicated in Figure 2). The total organic carbons (TOC) were consumed at an efficiency rate of 75%. In correlation, COD was consumed at an efficiency rate of 85% and the TPH was consumed at an efficiency rate of 99%. In addition, the phenols were also consumed to an extremely low level of 0.1 ppm (99% reduction).
The nitrogen uptake during the ACT process was particularly interesting in this project. As shown in Figure 2, the biological process alone (without any filtration), reduced the total nitrogen levels to a final amount of 50 ppm, indicating that 58% of the nitrogen was consumed by the bacteria. During the bioprocess, different forms of nitrogen were measured (ammonium, nitrate, nitrite, Kj-N, N-total).
It is important to note that the measurements were performed on mixed water from the bioreactor without any precipitation of the bacteria. Therefore, the difference of 68 ppm in the total-N levels before and after the ACT process represents the fraction that was evaporated from the system, and not assimilated in the bacterial cells.
The reduction of N-total is a consequence of a decrease in the nitrite from 56 ppm to 2 ppm. This is a very interesting finding because typically ammonia is a preferable nitrogen source over nitrite.
Apparently, the unique technology of ACT resulted in a bacterial cocktail that prefers nitrite as a nitrogen source. It is commonly known that de-nitrification occurs under anaerobic conditions, which raises the question of how such de-nitrification occurred in the continuously aerated bioreactor. However, this phenomenon of N-total reduction under aerobic conditions is field-proven, as demonstrated in this case. It has also been previously documented in scientific literature – Aerobic de-nitrification by a newly isolated heterotrophic bacterium strain TL1, T. Lukow and H. Diekmann, 1997. This paper indicates that this phenomenon is actually quite commonplace in sewage plants.
In summary, stripped sour water is a typical wastewater stream in refineries. Due to its contaminant profile and fluctuating nature, this stream is often the cause of upsets and overloads in the treatment process. In this article, the feasibility of the BPC-ACTTM process in treating such water is demonstrated. The biological process alone (without any post-treatment) resulted in a very efficient carbon and nitrogen uptake, with significantly lower sludge levels. The results clearly indicate that implementation of the BPC-ACTTM technology for stripped sour water treatment (either prior to the activated sludge stage or as a replacement to it) will lead to reduced overloads at an overall increased capacity lower operational costs.
Published: 10th Sep 2010 in AWE International