Sewage Chlorination Without De-chlorination: A Certified Impossibility

Credit: Marine Traffic
Credit: MarineTraffic

Published Mar 16, 2019 6:56 PM by Wei Chen

The approval regime has been in the blood of the marine industry. It has served well to safety equipment, but can the same be said when it comes to environmental technologies? An earlier publication suggested far reaching consequence of poorly enforced marine sewage rules. They have has gone beyond poor treatment performance status and started to erode the credibility of the approval regime. 

This joint publication explores one of the non-conformity issues in greater detail. The questions are simple: how can the approval assessment bodies have certified an impossible feature for sewage disinfection that they would not do for ballast water disinfection? How could “good test results” be obtained in the first place for impossibilities such as this and “no-sludge” claim to be certified?

Some may argue the existing sewage guidelines are vague and weak, but it is no excuse for the marine industry to be served with what is scientifically impossible. A certified impossibility need not happen in any industries, even without any guidelines. But, let’s hope to understand the root cause, so that corrective actions may be developed.

Sewage Treatment Plants Using Chlorination Disinfection without De-chlorination – A False Claim, and a Non-Conformity 

Co-authored by

Dr. Wei Chen, Future Program Development Manager, Wartsila Water Systems Ltd, UK 
Mark Beavis, IEng IMarEng FIMarEST, Managing Director, ACO Marine s.r.o., Czech Republic, ACO Marine Systems GmbH, Germany
Dr. Elmar Dorgeloh, Manager Director, Development and Assessment Institute in Waste Water Technology at RWTH-Aachen University (PIA), Germany
Felix von Bredow, Board of Hamman AG, Hamman AG, Germany
Endorsed by: 
Antony Chan, Engineering Manager, Victor Marine Ltd., UK
Dr. Daniel Todt, Project Manager R&D, Ecomotive AS, Norway
Helge Østby, Senior Technical Advisor, Jets Vacuum AS, Norway
Markus Joswig, Head of Marine Department, Testing Institute for Wastewater Technology GmbH (PIA GmbH), Germany
Benny Carlson, Chairman and owner, Marinfloc, Sweden
Tobias Kaulfuss, Manager - Marine Sewage Treatment, RWO - Veolia Water Technologies Deutschland GmbH, Germany
Greg Shannon, Technical Sales Director, JOWA AB, Sweden 

Chlorination has been used to prevent the spread of waterborne diseases such as cholera, dysentery, and typhoid. As one of the greatest advances of the modern era, it has saved millions of lives. Today, despite concerns regarding disinfection by-products (DBP) and the advance of other disinfection technologies, such as UV, it continues to be commonly used, especially when a residual is required to control the risk of microbial re-growth [1,2].  

Wastewater Chlorination 

The disinfection of wastewater has been extensively studied since the early 1900s [3] because of the potential exposure to humans via contaminated drinking water sources, recreation, and the consumption of fish/shellfish, etc. 

International shipping involves tens of thousands of ships drawing from and discharging into the sea (see map above). The sewage from ships includes that from onboard hospitals. It is only correct, therefore, to be ever mindful of the environmental impact and the risk of endemic diseases, not just in times of crisis. The coliform limits set in the marine rules are more stringent than those for equivalent coastal wastewater discharges from land.  

So, what does it take to reach the 100 counts/100ml limit specified in the MEPC.227(64) Guidelines? The effectiveness of chlorination depends on the wastewater quality, and the chlorine dosage that follows a time-concentration relationship. The removal of organics and particles from wastewater prior to disinfection is desirable if not essential [4,5]. Other influential factors include the pH, the temperature, and the presence of ammonia, etc. Wastewater can contain more than 106~107 counts/100ml of faecal coliforms. Conventional treatment processes can achieve 90 percent, or a one log, reduction. But a further 99.9 percent ~99.99 percent removal, or 3~4 log-kill, is needed. To achieve this, a chlorine dosage of 5-20 mg/l is required with a chlorine contact time of 30 minutes [4, 6]. 

Chlorination is only half of the job.


Since the 1970s, chlorine and other disinfectants have been found to form DBP that may be carcinogenic or harmful to the environment. Residual disinfectants themselves also cause harm to aquatic species. This has led to the adoption of a maximum residual chlorine target of 0.5 mg/l, as specified in MEPC.227(64). Although chlorine decays naturally, it takes many hours if not days [1,7]. 

The contact time in a sewage treatment plant is typically less than 30 minutes at its designed peak flow capacity. A chlorine concentration of 5-20 mg/l will not drop below the 0.5 mg/l limit in such a short time without a de-chlorination step. De-chlorination is a must prior to discharge. Otherwise, it is impossible to satisfy both microbial and residual chlorine limits, no matter how well the plant is operated.


However, de-chlorination is absent in some marine sewage treatment plants that use chlorination disinfection. The approval assessment bodies have accredited such equipment with IMO and MED certificates, based on “good laboratory results.” Thus, they have certified impossibilities. These “magic boxes” contravene science. These are non-conformities, and they turn certificates into licenses to pollute.

Over the years, such “magic boxes” have found their way onto many ships, contributing to the poor performance status of the sewage treatment plants. They set the “bar,” putting conforming technologies under “competitive” pressure by forcing them into a race towards the lowest levels of functionalities. In reality, coliform concentrations in treated effluent have been found to exceed the limits by a long way across all kinds of disinfection technologies employed in marine sewage treatment plants [8]. 

Ballast water management systems (BWMS), which perform less arduous disinfection duties than sewage treatment plants, may lend a useful reference. Chlorination-based BWMS have target chlorine concentrations ranging from 3 to 20 mg/l. Almost all of them incorporated de-chlorination prior to de-ballasting. Those that do not are subject to a certified minimum hold time of many days. The apparent inconsistencies between the approval processes of these two marine environmental products are hard to comprehend. 

There is a lot at stake. It may be time for the IMO, its Member States, and the assigned approval assessment bodies to acknowledge this issue, to undertake transparent and timely reviews, to identify the root cause and to prevent such non-conformities from reoccurring. 

For those who wish to be contacted for supporting further updates on this subject, please email [email protected].

[1] Wastewater Technology Fact Sheet – Chlorine Disinfection, USEPA (1999)
[2] Assessing the Need for Wastewater Disinfection. Haas, C.N., et al. Journal of the Water Pollution Control Federation 59: 856-64 (1987)
[3] The Disinfection of Sewage and Sewage Filter Effluents, Phelps, E. B., Water Supply Paper 229, United State Geological Survey (1909). https://pubs.usgs.gov/wsp/0229/report.pdf
[4] Wastewater Engineering, Treatment and Reuse, 4th ed, Metcalf & Eddy (2004)
[5] Guidelines for Environmental Management – Disinfection of Treated Wastewater, EPA Victoria, Australia (2002) 
[6] Wastewater Technology Fact Sheet – Disinfection for Small Systems, USEPA (2003)
[7] Technical Report: Chlorine Decay Study of Wastewater Discharges to Marine Waters from Stationary Small Commercial Passenger Vessels in Southeast Alaska, CH2M Hill, Alaska Department of Environmental Conversation (2006)
[8] Prevention of pollution by sewage from ships – rules and realities, Chen, W., ShipInsight (2018). https://maritime-executive.com/editorials/sewage-from-ships-rules-and-realities-1  

Dr. Wei Chen is Future Program Development Manager for Wärtsilä Water Systems.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.