The De Nora blog: Water Made Easy

De Nora Electrolytic Treatment vs. Traditional Biological Treatment

Aug 31, 2020 3:44:00 PM / by De Nora


Over the past decade, the industrial offshore and marine market space has shown an ever-increasing interest in more advanced, alternative treatment technologies to replace traditional biological wastewater treatment schemes. Yet, due to misinformation in the market and unwarranted finger-pointing, many industry decision-makers have yet to pull the trigger.

In response, our research experts have compiled this comprehensive comparison article to highlight the benefits of De Nora electrolytic treatment systems vs. traditional biological treatment methods.

An introduction

While many modern systems can successfully treat the sewage streams on marine vessels, offshore production platforms and drilling rigs, ease of disruption, and the effort needed to maintain these systems often prove to be more trouble than operators are willing to take on.

Historically, biological type treatment systems have been a favorite among large shipbuilders, smaller rig operators and small marine vessels where low capital expenditures on equipment drives purchases. 

Adding to this, set-to-sail schedules and hard-set delivery dates are often short, prompting the installation of the lowest cost, easiest-to-acquire treatment systems. 

Many of these systems are typically "priced to sell" and are often purchased to simply "check the box" on a project's overall scope of work. This scenario works out to be a positive for the shipyard building the ship or rig, but not necessarily for the end-user/customer of the marine vessel or the offshore drilling rig/platform operator.

Some real-world discoveries

In actual use, many biological systems reveal some troubling operational issues, as well as certain inherent shortcomings of the biological process itself. These issues are typically more apparent in the marine and offshore operating environment. 

It is not uncommon to see failures resulting from the poor maintenance of critical components, clogging (inlet screens not kept clear), abnormally high influent loading, loss of treatment-bacterial colonies and other issues leading to non-compliant effluent discharges. 

Over the years, real-world data collected by various government and independent agencies indicates that many systems simply cannot meet IMO (International Maritime Organization) and U.S. Coast Guard (USCG) certification duties due to the issues mentioned above. Some historical reports for reference can be found here:

Shining a light on common deficiencies

In recent years, we have seen more vessels being inspected and even detained by Port State Control (PSC) Authorities because of improper operation and maintenance of onboard sewage treatment systems. Some examples of common deficiencies found by the Singapore Maritime and Port Authority (MPA) are:

  • Untreated sewage being discharged overboard in port because the sewage treatment plant (STP) was bypassed or the overboard discharge valve was seized open or kept in an "open to sea" position
  • Aeration blowers were defective or not switched on when the STP is in use
  • Unauthorized modification to supply air to the STP instead of repairing the defective aeration blower
  • Level alarms or automatic operation of the discharge pump was defective
  • Improper operation and maintenance of STP, e.g., suction filter of aeration blower dirty/clogged, sludge return line (airlift) choked

Other common deficiencies noted in various reports are: 

  • When an STP is installed, it takes approximately 10 days before the bacterial growth in the bio tank becomes stable. During these 10 days, the STP doesn't perform sufficiently. Also, the STP units are often shut down when the vessel is outside the 12 nm limit (International waters), essentially starving the STP of the much-needed microbes to support proper treatment.
  • The shelf life of the chlorine tabs used for disinfection can be expired even before being introduced into the system, making adequate disinfection questionable. Oftentimes the chlorine tablets are not even present onboard.
  • Cleaning toilets with chemicals containing chlorine. If these kinds of chemicals are used, there is a strong negative effect on the biomass in the first compartment of the sewage treatment unit.
  • Insufficient maintenance relating to the cleaning of the sediment tank, i.e., checking the chlorine tablet levels and confirming there is sufficient aeration.

De Nora Omnipure 64 G2 electrolytic treatment vs. typical biological treatment   

While no treatment system is the absolute "be-all-end-all" for every application, general comparisons can be made to contrast the critical differences that separate a more advanced electrolytic treatment technology from a traditional biological type treatment system.

Durable, offshore industry quality components

Many Biological STPs utilize standard, non-industrial grade components to keep their costs down. Of course, this brings with it parts failures and potential discharge nonconformities, not to mention unscheduled maintenance of these components. All major process components and instruments on the OMNIPURE STP units are industrial grade and carry certain marine ratings.

De Nora's hazardous-area-rated treatment unit skids and electrical panels are IECEx and ATEX certified by Eurofins E&E CML Limited (CML) of the U.K. (IECEx notified body), and IMO MEPC.227(64) approved by the Bureau Veritas (B.V.).

Few manufacturers offer hazardous-area certification for their treatment units, and when included, are often on a project-to-project basis. Thus, passing the high-certification costs onto the customer.

At present, De Nora is the only major STP supplier to offer this dual-hazardous-area certification in OTS standard configuration. By absorbing this certification cost across the product line, De Nora can keep project costs low for customers.

De Nora's Patented DSA® electrode coatings

De Nora's Patented DSA® electrode coatings, showcases application-specific coating technologies that continue to advance the wastewater treatment segment.

A competitive footprint comparing equivalent/competitive "total equipment scope" requirements


OMNIPURE 64 G2 does not require externally sourced liquid sodium hypochlorite or chlorine bleach. Thus these materials will never need to be purchased, shipped, or stored. The Electrolytic process generates suitable chlorine residual in-situ during the treatment process itself. Other chemicals used in the treatment process are non-hazardous and can be stored on offshore facilities or aboard vessels with minimal health and safety (H&S) concerns.

A non-hazardous sodium sulfite powder is used after treatment to effectively neutralize the effluent stream before discharge tp sea.

A majority of biological STP systems use full-strength sodium hypochlorite/bleach as post-treatment disinfection, which can pose H&S concerns to crews on offshore platforms and marine vessels. Many times, the use of these chemicals are required to be reported on overall health, safety, and environment plans (HSE) for offshore platforms and marine vessels. These higher concentration chlorine chemicals also require dechlorination/neutralization before discharge to sea per IMO guidelines.

OMNIPURE offers a sanitary "post-treatment" wet solids handling system via an optional centrifuge unit. The wet solids produced by the OMNIPURE system don't require additional liquid bleach or further treatment to kill pathogens. This is because the wet solids discharged from the treatment unit still contain residual sodium hypochlorite, which serves as a further disinfectant within a customer's waste holding tank. Also, the wet solids collected from the batch tank contain the polymer used in the treatment process, which allows sludge thickening in the customer's waste holding tank. Often screening/filtering of raw influent sewage streams is required by many moving bed biofilm reactor(MBBR), membrane bioreactor (MBR), and biofilm-based biological treatment processes. The screened-out biomass/solids from filters/screens are still pathogenic and can cause severe health and safety issues aboard offshore facilities and marine vessels.


Biological treatment systems often require large tanks for primary & secondary treatment. An additional external tank is typically used for the collection of the hydraulic load. In some MBR based biological systems, a separate CIP (clean in place) tank is required for the membrane cleaning duties. The OMNIPURE Series 64 G2 treatment unit includes an on-skid, single-batch treatment tank, and requires an external tank for the collection of the hydraulic load.

Cost of operations

When it comes to the overall pricing of an STP system, it's easy to be swayed by a low-capital expenditure (CAPEX) and promises of maintenance-free operation. In truth, as the old adage says, you get what you pay for. Upfront costs are undoubtedly an important consideration, however, so are long-term operating expenses (OPEX).

  • Routine maintenance
  • System 'off-line' time due to membrane or tank cleaning
  • Power consumption
  • Long-term chemical costs
  • Health and safety expenditures 

In most cases, electrolytic type treatment systems are very competitive in the market space and offer apparent benefits. As De Nora has experienced on many occasions, replacing biological-type systems with newer, advanced-electrolytic technologies provides a client with a more manageable normal maintenance routine, lower chemical costs, lower energy costs and routine 'loggable’ compliance.


Most biological systems are simple 'flow through' devices that have difficulty coping with peak influent surges, or hydraulic overload conditions. This inherent design process can often lead to non-compliant discharges. 

The batch process of OMNIPURE 64 G2 presents a controllable "set" treatment process. Eliminating peak surge and overload issues, this feature facilitates a more reliable and effective wastewater treatment process. During each cycle, the treatment batch tank is flushed/washed with the sodium hypochlorite solution produced by the electrolytic cell. The entire tank is also emptied after each batch cycle, thus reducing any chances for septic conditions to occur in the tank. 

Many biological STP operators completely shut down their treatment units once they've entered international waters. It is typically required to have its aeration blower continuously running to sustain the system's microorganism colony, however, the absence of aeration activity in the bioreactor tank can cause the system to go septic during these non-operation periods. These septic conditions have the potential to produce toxic Hydrogen Sulfide (H2S) gas (rotten egg smell) in and around the treatment system. Besides being an inhalation hazard for operators, these gases also pose the potential threat of explosion. 

A notable report prepared by the Department of Defence of Australia highlights that when the aeration in a bio tank is turned off for approximately one (1) week, the sewage (sufficient sulfate and biodegradable material present) in systems on HMA ships would likely produce the H2S concentration. The risk is further increased in systems that use seawater flushing(3).


Once a typical biological treatment system loses its microorganism treatment colony, it can take days to reestablish the colony before effective treatment can again occur. During this time, untreated sewage is merely discharged into the sea. 

When a vessel with an electrolytic treatment system enters international waters, it is not required to be in full-time operation per the IMO as overboard pumping processes are allowed. Because of the active electrolytic treatment process of the OMNIPURE unit, effective treatment is easily controlled via on/off switch. When treatment is required, the unit is simply powered on, the wastewater stream is treated and is safely discharged into the sea. Specifically, for biological type systems, the condition of microorganisms during any non-operational period should be of concern and "on-demand" treatment is not guaranteed for many biological STPs. 

Because of the constant blower/aeration requirements, many Biological STP systems show high electrical power (24/7) usage. Since OMNIPURE system(s) are a batch-based system, operating "on-demand," electrical power is used sparingly when the unit is not actively treating.

Healthy microorganism populations

Biological STP system performance is significantly reduced when toilet cleaning chemicals containing chlorine or strong oxidizer chemicals enter the treatment system. Electrolytic STP systems do not rely on microorganisms in the raw sewage for the treatment. Consequently, standard cleaning chemicals that may be present in the raw influent sewage do not harm the treatment unit's performance. 

Electrolytic treatment units produce a lower, fixed wet solid volume after each batch treatment cycle. No field sampling or calculations are required. Some Biological STP systems often require returning activated sludge to the bioreactor tank as well as discharging 'wasting sludge' (dead microorganisms) to maintain a healthy microorganism population. If these steps are not followed correctly and the recirc sludge quality is not maintained, it can significantly impact the treatment performance of the STP. Sampling analysis and calculations for proper returning and wasting sludge is often required for some advanced biological type systems.  


Biological treatment systems often require cleaning and sanitizing of the bioreactor tank to avoid septic conditions. This process increases the downtime and interrupts the sewage treatment operation. In MBR and MBBR-based biological treatment systems, extra cleaning or replacement is often required for membranes and certain plastic biocarrier media.

The built-in, self-cleaning features of the electrolytic system are designed to maintain up-time and continued online service. Innovative reverse polarity electrolytic treatment cells and integral tank washing following each batch cycle also contribute to OMNIPURE 64 G2's consistent performance.

The OMNIPURE STP electrolytic cell can be effectively cleaned, when needed, with water and a non-metallic brush. On harder buildups, a diluted muriatic acid can likewise be used to effectively clean the cell’s electrodes. Separate neutralization activities of the diluted acid are not required as it is highly diluted before the cleaning task. Also, OMNIPURE treatment systems do not incorporate complex automatic or integrated acid cleaning systems, as this presents potentially dangerous H&S concerns due to the mixing of the acid(s) and chlorine solution. 

Most MBR-based Biological type systems require acid cleaning when trans-membrane pressures increase to preset levels. Typical acid (citric, oxalic, or muriatic) for organics removal, and oxidizers (sodium hypochlorite) for inorganics are routinely used. Due to the size of the physical membranes, the volume of acid used to clean them can be quite large. Additionally, complex systems are often required for neutralization after the cleaning process; these are typically hazardous and pose onboard storage as well as H&S concerns. 

Managing waste materials/health and safety

In general, the latest IMO MEPC.227(64) guidelines require the final effluent to meet such a low TSS (total suspended solids) rating — which is essentially clear water — almost all the TSS solids materials in the collected wastewater need to be removed from the treatment system. The OMNIPURE treatment systems reduce the level of TSS quite lower than the IMO discharge limit.

Operators must gain a clear understanding of the use and maintenance of raw influent screening/filtration devices before any marine STP operation begins. To clarify, sludge production, as mentioned above, and raw influent filtering are two completely different processes. 

Many/most biological type treatment systems require some sort of screening or filtering of the raw sewage inlet stream before any treatment takes place. This is especially true of many MBR and MBBR type biological systems as a means of preventing clogging and providing established bacteria colonies with an improved chance of destroying bacteria. At first, this may not appear to be a problematic situation; however, actual operator feedback from the field tells another story. The screened-out biomass/solids from filters/screens are still pathogenic and can cause severe health and safety issues aboard offshore facilities and marine vessels.

From an operator H&S standpoint, the problem here is apparent. Expecting operators to physically handle untreated, raw sewage should pose significant concerns for the companies who employ them. Odor aside, the risk of direct contamination to the skin, open cuts, eyes and mouth is quite high. As a result, many operators have become quite vocal about their adversity to performing such maintenance duties.  

Aside from the H&S aspect, customers should be wary of any manufacturer who promotes and sells an STP as a complete "sewage treatment unit" without adequately informing the customer of the need for additional tanks and/or inlet screens/filters. Without disclosing this pertinent information at the beginning of a project, the manufacturer has essentially done a disservice to their customer by offering a simple ‘raw-sewage-removal’ system, and not an actual "complete treatment system." Even though these types of biological systems may carry an International Maritime Organization (IMO) certification, the customers are left with having to manage untreated raw sewage separately from the purchased certified unit.

OMNIPURE offers a sanitary "post-treatment" wet solids handling system via an optional centrifuge unit. The wet solids produced by the OMNIPURE system don't require additional liquid bleach or further treatment to kill pathogens. This is because the wet solids discharged from the treatment unit still contain some residual sodium hypochlorite, which serves as a further disinfectant within a customer's waste holding tank. Also, the wet solids collected from the batch tank contain the polymer used in the treatment process, which allows sludge thickening in the customer's waste holding tank. 

Formation of disinfection byproducts (DBP)

The topic of disinfection byproducts (DBP) generated by the interaction of chlorinated effluent streams with organic matter in marine waters has gained much interest in recent years. DBP formation is an extremely complex and multifaceted reality of modern-day water treatment that is misunderstood by many. Yet, this phenomenon is presented globally under varying levels of expertise to the point that confusion and misunderstanding appear to be more prevalent than the level of DBPs themselves. 

Critics of hypochlorite-based electrolytic treatment systems often present the position that chlorine-treatment systems are incredibly hazardous to the environment and that chlorine(hypochlorite) use should be restricted or even banned from use. The same critics call for alternative oxidants-based treatment technologies such as chloramines, chlorine dioxide and ozone to decrease the formation of chlorinated DBPs; however, these alternative methods have been proven to produce other types of DBPs themselves. Presenting a fearful voice in the absence of factual analysis has proven counterproductive to understanding the cause of DBP variants and their actual effects on the environment. 

Chlorine has a long history dating back to the early 1900s(4) as a proven disinfectant in wastewater systems targeting human waste pathogenic organisms. It is considered the primary disinfectant in wastewater treatment systems worldwide.

Many biological treatment systems and other physical-chemical type STPs utilize concentrated commercial strength sodium hypochlorite in their disinfection scheme; therefore, effective neutralization of this chlorine is paramount in reducing the disinfection byproduct formation in the ocean environment. Systems with non-performing neutralization or dechlorination systems may contribute to higher DBPs in the environment beyond what typical electrolytic treatment systems produce. The careful selection of neutralization chemicals and neutralization injection rates are also essential to ensure that any chlorine discharge is at the minimally acceptable level set by the IMO.


To avoid many of the abovementioned operational issues, OMNIPURE™ marine sewage treatment systems effectively treat marine wastewater via a unique, patented oxidation process. Utilizing De Nora's patented electrolytic technology, the OMNIPURE system offers many advantages when compared to traditional biological treatment systems. 

While various chlorinated DBPs can be found in the environment following the expulsion of treated effluent discharges from traditional wastewater treatment plants, there is very little analytical data available regarding explicit exposure and health risks involving the saline seawater environment. Additional research and testing efforts should be undertaken to understand and categorize these specific DBPs fully. The OMNIPURE brand offers a level of advanced electrolytic treatment technology that is versatile and can adapt as regulatory requirements change.

Which Will You Choose?

Deciding between biological treatment and electrolytic treatment can be a daunting task. Get the facts before you commit to any installation. Download our FREE Comparison Guide to Total Cost and Ownership. Click below for more information.

Get the Guide

 *DSA is a registered trademark of Industrie De Nora S.p.A.


  1. Human Environment and Transport Inspectorate, Ministry of Infrastructure and the Environment, Rotterdam, the Netherlands, Report – Sewage Treatment Plants, Feb 2012 v.2 (online available at
  2. SRS e-Bulletin – 2016, Issue 01, Singapore Registry of Ships, The SRS' Growth in 2015 (online available at
  3. Lyn E. Fletcher, Potential explosive hazards from H2S production in ship ballast and sewage tanks, DSTO-TR-0750, Department of Defence – Defence Science and Technology Organization, Commonwealth of Australia
  4. Gordon G, Cooper WJ, Rice RG, Pacey GE. Disinfectant residual measurement methods. AWWA Research Foundation, American Water Works Association. 1987.


Tags: Sewage treatment, Offshore

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