Why a lack of alkalinity might be ruining your vessel’s waste treatment AND breaking regulations
In the operation and smooth running of a biological sewage treatment plant (STP) one of the parameters which is most often overlooked is alkalinity. This article looks at what alkalinity is, why it is especially important for on board systems, and what can be done to ensure that everything works as it should.
What is alkalinity?
In a water-based solution, the pH scale is used to describe whether the solution is acidic (pH of less than 7) or alkaline (pH of greater than 7). The pH is essentially a measure of how many hydrogen ions are in the solution. The more hydrogen ions there are, the lower the pH.
Alkalinity measures how effectively a solution can resist changes in pH, which would make the solution more acidic. It is very important to understand that although pH and alkalinity are linked, they are not the same thing. For example, imagine an acid is added to two beakers of water both with the same pH, but one with zero alkalinity and the other with high alkalinity. The pH of the zero-alkalinity solution will drop, whereas the pH of the high alkalinity solution will change very little, if at all. The reason for the lack of change of pH is that in the beaker with high alkalinity the hydrogen ions from the acid are absorbed by the alkalinity, and the pH does not change.
Alkalinity is measured by determining the amount of acid needed to lower the pH of a solution. This number is then expressed as an equivalent concentration of calcium carbonate (mg/l CaCO3), which essentially expresses how much calcium carbonate would be in the solution if it were the sole source of the alkalinity. But, although alkalinity is usually expressed in this way, it is important to realise that the alkalinity can be provided by chemicals other than calcium carbonate.
Why is alkalinity important when running a superyacht?
Biological systems are very sensitive to the pH of the solution in which they are in. Species such as proteins and enzymes will only work properly within a very narrow range of pH values, and can be very susceptible to any changes in pH. For example, the human body needs the pH of blood to stay between 7.32 and 7.42, and uses chemical buffers such as calcium carbonate to achieve this.
Biological wastewater treatment systems in STPs are no different. The bacteria and enzymes which treat the wastewater are also sensitive to pH, and operate most efficiently within the pH range 6.8 to 8.0. The biological wastewater treatment process also generates hydrogen ions, and so alkalinity is needed to keep the pH of the solution in the required range. If the alkalinity is too low then the extra hydrogen ions are not removed, the pH drops, and the speed of the wastewater treatment slows or even stops. If the STP is nitrifying too (converting the ammonia in the wastewater into nitrates) then this process will also consume alkalinity. Approximately 7g of alkalinity (as CaCO3) are consumed for every 1g of ammoniacal nitrogen.
This makes it even more complicated, as not only is it does the user need to know the alkalinity of the wastewater prior to treatment, it is also helpful to measure the residual alkalinity of the treated wastewater. The leftover alkalinity is a useful measure of whether the STP’s biological processes are working efficiently.
Impact on vessels
Alkalinity in natural water comes from the soil and rocks, which the water passes through prior to treatment. This means that the alkalinity present in shore water will vary depending on the geology of where the shore water was treated.
It also means that when a vessel changes from shore water to water produced on board (through reverse osmosis), the process will remove the majority of the alkalinity from the water. This has a dramatic effect on the pH, and therefore the performance, of the STP.
As any discharges from a superyacht must comply with MEPC.227(64), this process could also mean owners are breaking the law if not aware of its effects. MEPC.227(64) states that ‘The pH of the samples of effluent taken during the test period should be between 6 and 8.5.” Ensuring an adequate alkalinity concentration in the wastewater will enable the pH to be kept within this range, and so will keep any discharges compliant with the legislation.
How AFECO can help
AFECO can provide expert process advice, management packages and compliance checks covering the performance, operation, and optimisation of STPs. This will ensure compliance with the MARPOL regulations, including engineer’s training and support, meaning one less thing to worry about onboard.
As part of our evaluation of a vessel’s STP we can measure alkalinity of both the wastewater entering the STP and the treated water leaving the STP, to determine whether the alkalinity concertation is high enough for efficient and compliant operation.
If the alkalinity is too low then not to worry; there are options for how this can be corrected. We supply a product called biobuffer, a proprietary alkalinity-raising filter system that is tailored to suit the flow requirements of an individual system. Water is passed through the filter system before entering the biological treatment process, saturating the water with sufficient alkalinity to maintain the conditions required to support a healthy biological treatment system.
We also supply a liquid alkalinity product called biobuffer liquid SC010, which can be dosed into biological treatment systems as required, for example upon the switch from shore water to water produced on board, to provide sufficient alkalinity.
We are completely independent of all manufacturers and suppliers of wastewater treatment packages, meaning we are able to provide truly impartial advice and support. We are experts in biological, semi-biological and chemical wastewater treatment systems, and have a wealth of experience in wastewater treatment, spanning over 20 years.
We will provide you with the tools required to ensure your sewage discharge remains compliant with Legislation and your guests can get the most out of their experience on board.
If you would like to find out more about how AFECO can support you please get in touch at info@afeco.co.uk.
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