The De Nora blog: Water Made Easy

Marine Sewage Treatment Plants for Offshore O&G: The Dilution Falsehood

Sep 17, 2020 9:00:00 AM / by Dana Casbeer


When it comes to water treatment, the adage “dilution is the solution to pollution” may not be correct, especially when it comes to the International Maritime Organization’s (IMOs) marine sewage treatment plants (STPs). For years, this snappy reference has been shouted by countless environmental groups as well as marine wastewater treatment professionals. So much so that the real sense or intent of the saying is what has indeed become diluted. 

In fact, many that shout this ‘proverb’ may not necessarily understand the real rationale behind additional fluids being introduced into specific wastewater treatment processes. Thus, making the sweeping argument that all dilution is somehow ‘cheating’ the system is merely misguided intent.

Water, water everywhere

In the rawest sense, it should be noted that the addition of any fluid(s) into a marine sewage treatment system that is not the actual waste product itself (e.g., sewage(black water) and greywater entering the STP’s main collection tank) can be considered dilution. But what many don’t realize is that per the Marine Environment Protection Committee (MEPC), greywater (those water wastes from sinks and showers) is also considered ‘dilution’ water if introduced into an STP system after the main wastewater collection tank. 

That’s right, the simple addition of waters from the sinks and showers entering an STP unit after the influent sample point(Ci), is considered dilution per the MEPC’s latest guidelines. The introduction of the dilution compensation factor (Qi/Qe) takes into account this additional dilution water(Qd). It provides the equipment manufacturer a required proportional reduction value that must be considered for the ‘certifiable’ effluent variables in the actual IMO testing protocol.

In this case, the apparent rub comes into play when this Qd variable is something higher than a value of one(1), resulting in a high total Qe effluent flowrate. When this dilution flow is much greater than the inlet wastewater flow, the resulting derating factor essentially drives the required effluent discharge value to such a low level that many systems can’t show feasible treatment.

In reality, many approved STP systems on the market show this Qd value as being extremely high, which makes the final effluent Qe measurable effectively skewed due to the abundance of dilution water. Also, it should be noted that during any prescribed IMO efficacy testing for certification, this additional water(Qd) must be measurable and controlled. In the case of greywater from sinks and showers entering an STP after the initial waste stream collection point, there is no feasible way to control these flows. In most cases, these are gravity drains, and the daily volume can be exponentially high, depending on the overall treatment system configuration. This can be seen in a few MEPC.227(64) Type Certified units currently on the market today and is one of the key factors driving this entire debate on the addition of dilution water.

Sticking to the rules — naysayers aside

There is no specific notation banning, or eliminating the term dilution in the latest IMO MEPC.227(64) wastewater discharge guidelines. There is also no mention of, or instructions to cease or eliminate any or all additional non-wastewater streams from entering the STP unit being compliance tested per MEPC requirements.

Those of us with years of real-world wastewater treatment experience look at this MEPC position as one of having a solid understanding of the available technologies and treatment methods needed for the effective treatment of marine wastewaters. Most of today’s technologies rely on some amount of additional waters to operate effectively and meet the already scrutinous effluent discharge requirements. If the MEPC were to take a stronghold position to eliminate all dilution water/fluid from entering an STP unit, a high percentage of current STP original equipment manufacturers (OEMs) would have to adopt alternative treatment technologies or cease to exist.

Again, the devil is in the details. The MEPC guidelines state clearly that an approved sewage treatment plant should not rely solely on dilution of wastewater. Words have meaning. The fact is dilution is allowed per the IMO, and those OEMs who manufacture treatment systems that require a certain amount of additional waters for their systems to perform properly may use additional waters if they comply with the prescribed formula laid out in the latest Marine Environment Protection Committee (MEPC) guidelines. As they say, the proof is in the pudding, or in this case, the effluent sampling.

Recently, several marine wastewater treatment professionals have raised concerns regarding the marketing of many STPs. Touted as “magic boxes”, these ads promise unbelievable performance in a package as small as a traveler’s suitcase. This criticism is rightly justified in those examples where an STP’s dilution water port is two to three pipe sizes larger than the sewage inlet port.

Unfortunately, many end-users (customers) never question, nor have an understanding, that these sorts of dilution machines don’t ‘treat’ the sewage wastes. When manufacturers skirt the rules in this manner, the competitive landscape is skewed and introduces many questions to the validity of authorized Notified Body(NB) 3rd party testing. Once a customer receives his/her IMO Type Approved/Certified STP, he/she rarely questions the treatment validity. He/she truly believes that they are environmentally conscious when using the product.

Treatment Trumps Dilution

There are, however, many Type Approved/Certified marine STP units available today that do adhere strictly to the latest MEPC guidelines while utilizing a minimal amount of dilution water to maintain effective sewage treatment. Effective, analytically proven treatment — not dilution — is the solution to pollution. The key terminology to emphasize here is ‘treatment’. The term treatment holds a specific meaning in the grand scheme of wastewater effluent quality, with regards to IMO regulatory compliance and is analytically measured for effectiveness.

Today, many electrolytic treatment systems, such as De Nora Water Technologies (DNWT) latest patented electrolytic-oxidation unit, require a seawater-to-sewage ratio of less than 1.0 and utilize an electrolytic treatment cell that permits more thorough oxidation, or ‘kill’ of the pathogenic material insitu while treating.

In addition, DNWT’s electroflotation process effectively separates the sanitized total suspended solids (TSS) material, producing quality effluent discharge well below the latest IMO requirements. These effective treatment characteristics would not be possible without the addition of supplemental seawater.

For any STP attempting to meet the latest IMO discharge standards, testing of its requisite samples must be tightly controlled, witnessed by a 3rd party, and be confirmed through an approved testing laboratory. 

STP systems that are genuinely treating the wastewater will show a significant decrease in bacterial counts, as well as particular organic and chemical oxygen demands. More advanced STP systems will actively destroy these pathogenic bacteria, rendering their further damaging effects on the environment mute.

On the flip side, those STP systems that use high dilution water flow rates to sidestep the treatment process are only submitting effluent samples to the lab, which results in testing more of the dilution water itself than the accountable pathogenic material in the wastewater stream. These types of STPs essentially allow viable bacteria and/or pathogens to be washed out to sea to continue their lifecycle.

Don't Wait Until It's Too Late

Be sure you fully understand the capabilities of a water treatment system before you commit to its installation. Download our FREE Guide to Marine Offshore Sewage and Water Treatment Options. Click below for more information.

Get the Guide


Tags: marine, water treatment, Sewage treatment

Dana Casbeer

Written by Dana Casbeer

Lists by Topic

see all

Posts by Topic

See all

Recent Posts