The De Nora blog: Water Made Easy

When Marine Sewage Treatment Systems in Offshore Oil & Gas Operations Don't Really Treat

Sep 22, 2020 3:56:00 PM / by Dana Casbeer

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The real-world implications of marine wastewater pollution can have a devastating effect on the marine ecosystem and human health. Although not a subject that many would discuss over dinner, some of us find it reassuring that the marine equipment we use is carefully designed for the preservation of the marine environment. 


As environmental scrutiny of the Offshore Oil and Gas (O&G) and the maritime vessel wastewater treatment sectors intensifies globally, it has become evident that the way marine sewage treatment plants (STPs) are designed, third-party certified and operated is drawing much attention and debate from end-users, as well as certain environmental groups.

This year marine port authorities (MPAs) from Ghana and Singapore released strict warnings to the shipping world that pollution from STPs specifically, in their harbors and coastal waterways, will not be tolerated. These countries are now actively detaining vessels and issuing fines for polluters who release non-IMO (International Maritime Organization) compliant discharges into the sea. While offshore O&G facilities may avoid such high visibility due to their remoteness out at sea, there is still great risk to operators who are found to be ‘skirting the rules’.

This trend will likely grow at a steady pace as more countries become active stewards of their local marine ecosystems, realizing that the health of these systems is paramount to the livelihood of their landlocked inhabitants as well as their economic wealth. Hats off to those in regions that are taking these proactive steps in monitoring their waterways and establishing a robust environmental stance for the preservation of marine waters.

Answering the tough questions

There comes the point when end users (our customers) begin to ask some simple but fundamental questions regarding IMO marine sewage treatment systems. Such as, “Why do so many STPs have problems meeting the regulations?”, ‘How can such a small IMO treatment unit handle my daily treatment needs?’” and lastly, “Why did I pay for an IMO-certified sewage treatment system if I still have to manage a completely separate raw solids waste holding tank filled with raw sewage?’ 

These are great questions that showcase the recent heightened awareness of technical and environmental issues shared by our customers. Today, those in search of suitable IMO-compliant treatment systems are asking original equipment manufacturers OEMs of so-called “certified” marine STPs these critical, direct questions.

As IMO wastewater effluent discharge regulations tighten, STP owners and operators have reached a heightened level of knowledge, awareness and responsibility that goes beyond cost. Understanding the capabilities of available treatment solutions and sludge-handling options allows these customers to make educated, long-term equipment selections based on their specific operational areas and company policies.

A false sense of treatment

There is a myth afoot that all type-approved marine sewage treatment plants are created equal and capable of treating sewage. In many instances, this is simply not the case. In many instances, this is simply not the case. Since crew safety and operation maintenance requirements are often key purchasing factors, it begs this question from a health, safety, and environment (HSE) perspective:

If these internationally-regulated STPs have been so rigorously examined and tested to meet the IMO discharge standards, why are some systems permitted to technically remove the bulk of the actual raw biomass(sewage) through screening or filtering, thereby allowing a ‘cleaner’ wastewater stream with lower biological oxygen demand (BOD) and total suspended solids (TSS) load to enter the treatment unit? 

The removal of the influent biomass indeed reduces the overall organic load on the STP, while still producing an IMO acceptable discharge. But, in many cases, this reality is a significant disservice to customers. Why? It imposes an entirely new set of logistical handling procedures, as well as health and safety issues for operators.

In a perfect world, sewage treatment means being able to treat the entire wastewater hydraulic load, without having to first filter out raw sewage-laden solids; mass that will need to be dealt with by additional processes (many times manual removal) outside the IMO compliant equipment scheme. Many operators will attest to the fact that STP downtime and the physical workforce required to shovel out sludge-laden holding tanks bring about a psychological perception that all STPs are more trouble than they are worth.

Not explained by many suppliers, the introduction of waste stream liquids, such as galley greases and food scraps (from InSinkErator® or macerators), can also harm many STP units if not separated and removed via a suitable grease separator. Undigested food scraps and shredding add an extremely high BOD load (in the thousands of mg/L) into an STP unit and can lead to unwanted blockage and increased corrosion. This load causes the system to treat ineffectively, thus allowing high BOD levels to exit the unit in the effluent discharge. That said, permitting food waste scraps and shredding into the STP is highly discouraged by knowledgeable suppliers and the IMO as well.

While there are particular sludge dehydration, thermal drying, and biogas generation systems available to ease the burden of collected wet wastes, many are not feasible for offshore marine installations; thus, additional high-maintenance equipment arrangements must be considered, increasing capital expenditure (CAPEX) and operational expenditure (OPEX) costs.

Because many STP treatment technologies available today simply cannot adequately treat the biosolids influent loads, logistical removal, handling and transportation of sewage sludges, these issues will continue to plague many installations until customers begin to seek out and demand more specialized STP treatment systems.

From vessels to fixed Offshore O&G installations – the logistics differ greatly

In the offshore oil and gas (O&G) sector, the major exploration and production (E&P) players fully understand the adverse financial effects of production downtime and the inevitable tarnished reputation that failed STPs and noncompliance can cause. Having a properly operating STP onboard a fixed-position, offshore platform requires an in-depth understanding of the unit’s operational requirements, maintenance duties and waste handling procedures.

Under MARPOL 73/78 Regulation 11.1.1 allowances, the IMO currently allows sewage wastewater to be pumped directly into the sea from moving vessels. However, this regulation does not carry over to fixed or floating offshore oil and gas installations. On these installations, any discharged sludges or other wet wastes from an STP unit must be managed onboard and kept under careful watch, so as not to turn ‘septic’ and produce dangerous hydrogen sulfide (H2S) gas. Besides being an extreme health and safety (H&S) issue, the H2S interaction with moisture facilitates the formation of sulfuric acid and presents a dangerous corrosion risk within tanks, pumps and piping systems.

Physical space on offshore industrial platforms is at a premium and reserved for actual ‘money-making’ operations. Large wet solids/sludge holding tanks not only take up prime real estate but also add another layer of weight and stability issues for floater units in particular. Furthermore, a platform’s wet waste/sludge tank could become extremely large in volume due to the required holding times needed to accommodate many offshore supply vessel’s (OSV) schedules. The longer it takes an OSV to service the platform, the more volume capacity is needed for such tanks.

Lastly, if the tank is not properly designed with adequate ventilation and recirculation, it can easily produce the unsafe H2S conditions described above. This leaves offshore E&P operators few options for handling sludges or bio-solids while maintaining operational status aboard the platform.

It’s often easier said than done

Offshore and marine wastewater treatment offers many challenges for OEMs and the operators of STP equipment. In both ocean-going vessels and fixed O&G platforms, the problems can quickly add up. Remember, from the hydraulic load-wise point of view, operators are essentially looking at treating the equivalent of a small, onshore, residential community of 100+ people in the footprint of a small kitchen.

As mentioned above, on offshore O&G production facilities, there is usually little physical space allotted for the STP installation. This is due in part to STPs not being included in the design phase of older vessels or platforms. To further complicate matters, the space allowed is configured in such a way that a standardized STP design provided by most suppliers simply will not fit without significant modifications to the deck or compartment.

Next, one needs to consider the sheer hydraulic load requirements realized at any particular installation. Often, human sewage and grey waters are not the only waste directed into STP systems. This combined waste stream can create havoc with sensitive treatment process designs. Overload influent conditions are so common that many customers opt to include large overflow piping schemes into their STP systems’ inlet collection tank. This piping then facilitates the overflow of raw, untreated sewage in the event of high-hydraulic load periods. Spilling of raw sewage overboard during these high surge periods leads to other problems on the environmental front.

Lastly, we must consider additional treatment system-related equipment or components (outside of the STP configuration) that must be present for the overall treatment arrangement to perform appropriately.

In light of these challenges, some STP systems are making their way into the marketplace that promise complete wastewater treatment and trouble-free operation.

Separating reality from fiction

Recently, many of those knowledgeable in the industry have cited certain nonconformities related to various STP treatment processes under MEPC.227(64). Their analysis indicates that there are several manufacturers of STPs who have seemingly rampant disregard for proper IMO marine sewage treatment unit design and/or class society performance testing.

While the allowance of such process nonconformities could be placed on various member societies of the International Association of Classification Societies (IACS), many are a direct result of shortcomings in the STP design itself, simply to fit into a specific installation space. Many times, these nonconformities can result from an overabundant use of dilution water — either seawater or grey waters fed into STP systems (pre and/or post-treatment) — that can positively skew the effluent discharge analysis. In reality, much of the harmful pathogenic bacteria bypass any true treatment, and these solids are carried into the sea with the effluent stream.

Other STP systems simply claim to be able to meet IMO standards without any sludge production, while the unit itself is no larger than a suitcase. Units of this minimal size are such a departure from the basic tenets of science-based wastewater treatment principles that it is troubling to find them in the mainstream equipment offerings. It is also doubtful that specific IACS member societies duly approved these substandard designs.

Adding to the confusion, there are several copy-cat STP units on the market today that closely mimic the mechanical design and treatment process of older De Nora Water Technologies (DNWT) MEPC.2(33 CFR 159) compliant electrolytic STPs. While their designs are based on dated technology, these STPs have been granted IMO certifications to the latest MEPC.227(64) performance requirements. This compliance is certainly questionable for DNWT (the original Certification holder) as they thoroughly understand the limitations and actualities of the treatment processes used in these subject systems.

 De Nora Water Technologies’ STPs have never required filtration, screening, or bulk biomass removal of raw waste streams before treatment. Fine maceration of the influent waste stream allows for increased surface area exposure of the wastewater within our patented electrolytic treatment cell, permitting more thorough oxidation, or ‘kill’ of the pathogenic material insitu while treating. Combining our electroflotation process effectively separates the sanitized TSS material, producing quality effluent discharge well below the latest IMO requirements.

Most of the total suspended solids (TSS) encountered by STPs are made up of cellulose fibers in toilet tissue. This tissue is also the primary cause of plugging within specific STP systems. If not comminuted or macerated effectively, the cellulose fibers will tend to agglomerate, producing larger clumps that are often difficult to flush from the treatment system and ultimately need to be removed manually.

Although advertised as “flushable,” wet wipes should never be introduced into any marine STP unit. Just ask the chief engineer or equipment technician responsible for clearing the plugged STP. Cellulose is insoluble in water and therefore remains as a suspended solid (SS), of which a high percentage must be physically removed from modern STP units to comply with current discharge regulations.

While electrolysis alone does not destroy the entire TSS load in the wastewater stream, a certain degree of breakdown is achieved within the DNWT patented processes. At the same time, electroflotation manages any TSS, preventing carryover. Where preferred, an optional centrifugal separation process provides a suitable Class ‘B’ landfillable solids waste discharge that eases the customer’s concerns of managing auxiliary tanks of raw, pre-screened sewage wastewater solids.

Does Your Water Treatment System Have What It Takes? 

Be sure you fully understand the capabilities of your water treatment system before it’s too late. Download our FREE Guide to Offshore Sewage and Water Treatment Options. 

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Disclaimer: The views and opinions expressed herein are mine and do not necessarily reflect any views of any entity the writer has been, is now or will be affiliated with. 

Tags: marine, water treatment, De Nora, Sewage treatment, Offshore, maintenance

Dana Casbeer

Written by Dana Casbeer

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