Microbiologically influenced corrosion (MIC) is a major integrity threat for water treatment systems as well as other utility water systems. Erroneous chemical treatments associated with MIC control and mitigation process further exacerbate MIC issues within such systems. This article investigates such chemical treatment shortcomings and their main root cause(s).
Water treatment systems often play a crucial role in many production assets by supplying various utility systems with treated water. Simultaneously, MIC is a major integrity threat for both water treatment and utility water systems.
To ensure continuous and smooth operations, the MIC threat must be mitigated efficiently. MIC mitigation is often achieved through chlorination and biocide treatments. However, various shortcomings associated with these treatments often significantly undermine their effectiveness, leading to more severe and widespread MIC issues and failures.
The main objective of this article is to share such shortcomings and their root cause(s) with other operators and asset owners to avoid repeating the same mistakes or following similar erroneous chemical treatment procedures.
Water Treatment Systems Overview
The main function of a typical water treatment system is to take raw water from a water source and then to treat it in such a way that the treated water phase meets a specified or accepted treated water specification. The principal objective for treating the raw water phase is to either eliminate or minimize the corrosion integrity threat posed by various raw water sources.
Raw water sources, depending on the asset location and operators’ preferences, include:
- Aquifer and well water;
- River water;
- Lake, sea, and ocean water.
After the received raw water is treated, the treated water is often used by various utility water systems located further downstream. Such utility water systems include:
- Sea water injection;
- Fire water;
- Sand wash water (for separators where sand may accumulate at the bottom half);
- Cooling water;
- Heating water (or medium);
- Product displacement water.
Any failures or shortcomings in adequately treating the raw water phase to meet the required treated water specification would have serious and significant detrimental repercussions for the aforementioned utility water systems.
MIC Control, Mitigation in Water Treatment
Oxygen corrosion and MIC are the two main corrosion damage mechanisms associated with not only the water treatment systems, but also with the aforementioned utility water systems located further downstream. However, in this article only the MIC damage mechanism, its control and mitigation, and finally the shortcomings associated with such control and mitigation measures are covered and discussed.
Bacteria often enter most water treatment systems through their inlets from either of the raw water sources already listed. Provided that favorable nutrition and growth conditions exist within such water treatment systems and further downstream, the bacteria will proliferate and multiply within these systems progressively and increasingly. This phenomenon could lead to severe localized corrosion rates, often at the bottom of equipment.
To control and mitigate MIC, most water treatment systems rely on a two-stage chemical treatment procedure. That is, chlorination at the raw water inlet or immediately downstream of the inlet followed by biocide treatment further downstream–often at or downstream of the deaeration equipment.
Ideally, an adequate, consistent, and continuous chlorination treatment is followed by bespoke batch biocide treatment. Simultaneously, both planktonic and sessile bacterial sampling and analysis must take place regularly, both prior to and after such chemical treatments, to assess their effectiveness. Obviously, any shortcomings associated with either chlorination or biocide treatment or both would have severe detrimental repercussions for all water systems involved.
The author’s experience for the past 20 years suggests that most water treatment systems visited or studied often suffered from inadequate, erroneous, or total lack of an appropriate chemical treatment for effective MIC control and mitigation, as detailed in the following section.
Encountered Chemical Treatment Shortcomings
The author has been cataloguing chemical treatment shortcomings associated with MIC control within water treatment systems since October 2002.
For water treatment systems, such shortcomings are divided into the following two categories:
- Chlorination treatment shortcomings
- Biocide treatment shortcomings
Both categories are presented below in the form of two separate lists covering various chemical treatment shortcomings encountered or studied in the past 20 years.
Operators and asset owners are encouraged to study both lists carefully to ensure that similar shortcomings do not exist in their assets or are rectified and fixed immediately if they existed in the first place.
Chlorination treatment shortcomings include:1
- Carrying out no chlorination treatment at all;
- Carrying out batch or intermittent chlorination treatment instead of a continuous one;
- Carrying out chlorination at alkaline pH values, where the oxidizing power of chlorine, hence its killing power, significantly decreases;
- Adding chlorine at a location where it does not mix adequately with inlet water;
- Adding chlorine at an inadequate concentration, leading to inadequate residual chlorine levels in the system;
- Adding chlorine; however, with inadequate residence time. Thus, the added chlorine does not have enough time to achieve a complete or more efficient planktonic bacterial kill;
- Adding chlorine downstream of the oxygen scavenger injection point. This practice renders the injected chlorine useless, since it reacts and is deactivated by the present oxygen scavenger chemical;
- Carrying out continuous chlorination; however, for only a small portion of the inlet water. This often happens in facilities that possess rather huge pools for holding water along with multiple chlorination treatment points, of which only one or a few function properly on a continuous basis and most of the injection points are somehow out of service;
- Carrying out chlorination and biocide treatments at the same location where their injection points are located at proximity with each other;
- Carrying out chlorination at certain locations to kill sessile bacteria and not planktonic bacteria, and chlorine is not effective against sessile bacteria or biofilms.
Biocide treatment shortcomings include:1
- Injecting the biocide chemical at such a low concentration that it does not kill the existing bacteria;
- Injecting a biocide chemical that is only capable of killing planktonic bacteria and is ineffective against sessile bacteria and biofilms;
- Injecting a biocide chemical at a wrong frequency, rendering it rather inefficient against the existing bacteria;
- Injecting biocide at such operating conditions that can deactivate or decompose it, such as injecting at operating temperatures that are higher than the maximum operating temperature specified for that particular biocide;
- Injecting a biocide chemical that lacks chemical compatibility with other injected chemicals and is thus neutralized upon injection;
- Injecting biocide at the wrong location, e.g., much further downstream of the required location, thus leaving the systems and equipment upstream of the injection point unprotected against MIC;
- Injecting a biocide chemical for such a long time that the target bacterial populations gradually become immune against that biocide;
- Injecting biocide at the wrong pH value or the wrong pH range, as some biocide chemicals perform best at certain pH values or pH ranges based on their datasheets;
- Injecting biocide chemicals that lack surface active agents and thus cannot penetrate biofilms to kill the sessile bacteria underneath the biofilm;
- Injecting biocide upstream of the oxygen scavenger chemical instead of injecting it downstream of that chemical.
Main Root Causes of the Encountered Shortcomings
Inadequate or total lack of competence in relation to bacterial and MIC basics has been the main culprit behind the majority of the encountered chemical treatment shortcomings.
In one case, which is also believed to be the worst MIC case ever observed by the author, in terms of the shear magnitude of the financial losses incurred due to the widespread and increasing MIC leaks and the corresponding repairs. The water treatment plant manager had reasoned that because bacteria were too small to be seen with naked eyes, the integrity threat they posed was accordingly negligible. Thereafter, he had ordered the biocide treatment to be completely stopped! After two years, the ensuing repair costs were significantly greater than $100 million.1
- A significant portion of the observed MIC failures and leaks was due to chemical treatment shortcomings associated with the MIC control and mitigation process.
- The main root cause or culprit of such shortcomings was the total lack of or inadequate competency in relation to bacterial and MIC basics among relevant personnel.
- To avoid repeating the same mistakes, operators are encouraged to carefully study the above two lists associated with chlorination and biocide treatment shortcomings and ensure that similar mistakes are not repeated within their assets.
- Relevant personnel are recommended to attend MIC training courses to improve their knowledge and competency, namely in the following four areas:
- Bacterial nutrition and growth conditions
- Bacterial and MIC monitoring
- Bacterial and MIC assessment
- Bacterial and MIC control
Editor’s note: This article first appeared in the August 2023 print issue of Materials Performance (MP) Magazine. Reprinted with permission.
1 Morshed, A., A Practical Guide to Microbiologically Influenced Corrosion (MIC) Management in the Upstream Oil and Gas Sector, (Houston, TX: AMPP, 2022).