Dometic Max Control Natural: How It Works and How to Use It

You’ve added holding tank treatment and flushed the toilet. That should help control the odors, right? Depending on your boat’s sanitation system, the treatment may still be sitting in a vacuum generator or discharge line instead of reaching the holding tank.

Dometic Max Control Natural® helps control odors and break down waste, but getting the product where it needs to go matters. Here’s how the formula works, when to add it, and what to know about using it in your boat’s system.

How Max Control Natural Controls Holding Tank Odors

As waste breaks down in a black water holding tank, bacterial activity can produce unpleasant gases. Anaerobic bacteria, which thrive without oxygen, are associated with the strong, sulfurous odors familiar to many boat owners.

Max Control Natural uses an enzyme and bacteria formula to help control odors and break down waste solids. That breakdown also helps reduce the sludge that can accumulate inside the tank over time.

The product has a light fragrance, but its purpose goes beyond covering up a smell. It supports biological waste breakdown as part of your holding tank maintenance routine.

How Max Control Natural Differs from Earlier Treatments

Earlier Max Control treatments contained bronopol, a chemical used to control bacteria. Bronopol is classified as very toxic to aquatic life, raising environmental concerns about its use in waste treatment products.

When choosing a holding tank treatment, consider its ingredients and disposal instructions. For guidance on managing sewage from your boat, visit the EPA’s vessel sewage discharge resource.

Max Control Natural takes a different approach, using enzymes and beneficial bacteria to help control odors and break down waste. It contains no formaldehyde or bronopol, and Dometic states that the formula supports waste breakdown in downstream septic and municipal treatment systems.

Not sure whether a holding tank treatment is compatible with your sanitation hoses? Contact Environmental Marine to confirm the treatment and hose manufacturer’s recommendations.

Liquid or Dissolving Packets: How Much Should You Use?

Max Control Natural is available in two forms:

• Liquid holding tank treatment: Dometic lists a dose of 4 ounces per 40-gallon tank.
• Water-dissolving packets: Dometic lists one packet per 40-gallon tank.

Check the directions on your package for dosing, water requirements, and application instructions. The liquid requires measuring, while the packets provide a premeasured dose.

Add Treatment After Emptying the Holding Tank

Environmental Marine recommends adding Max Control Natural after pump-out, when the tank is empty. If the tank needs rinsing, complete that step before adding the treatment.

For most boat owners, adding the product through the toilet is the simplest route:

1. Empty the holding tank, and rinse as needed.
2. Add the appropriate dose through the toilet using water as directed on the package.
3. Flush enough to carry the treatment into the holding tank. The number of flushes depends on the sanitation system and plumbing layout.

That last step is easy to overlook, especially with a vacuum toilet.

Using Max Control Natural in a VacuFlush System

In a VacuFlush or similar vacuum toilet system, waste passes through a vacuum generator before reaching the black water holding tank. Treatment added through the toilet follows the same route.

Environmental Marine recommends two to three flushes total to help move the product all the way into the holding tank. The first flush carries the treatment into the vacuum generator tank; another one or two flushes help move the product into the black water holding tank.

The important point is that an empty toilet bowl doesn’t necessarily mean the treatment has reached the holding tank. Not sure which equipment you have? Use our vacuum generator, tank, and pump identification guide, or contact us for help determining the appropriate flushing procedure.

Other Toilets: Check the Distance to the Tank

For other toilet types, we advise that additional flushes generally aren’t needed unless the toilet is more than approximately 15 feet, or 5 meters, from the holding tank.

A longer plumbing run can leave treatment in the line before it reaches the tank. The amount of water used per flush and the system layout also matter, so treat that distance as a practical guide rather than a fixed rule for every boat.

Can You Add Treatment Directly to the Tank?

Depending on your installation, there may be two other ways to add treatment:

• A designated tank inspection port: Treatment may be added directly if the tank is equipped for this access and the equipment instructions permit it.
• The deck pump-out fitting: Liquid treatment may reach the tank through this fitting on some boats, but a check valve in the pump-out line can block its passage.

Don’t assume that pouring liquid into the deck fitting delivers it to the tank. If you don’t know whether the line contains a check valve, ask us about your installation before going on that route.

What If Odors Continue After Treatment?

First, check that you used the recommended amount and flushed enough to deliver it to the holding tank. If odors remain, note where and when you notice them.

A persistent smell inside a compartment can point to a connection problem or a dirty gray water sump. Odor outside immediately after flushing may be coming from air leaving the holding tank vent. More treatment won’t repair a leaking connection or clear an obstructed vent.

For odors carried outside through the vent, a properly selected filter may help. Read How Vent Filters Help Control Holding Tank Odors on Boats to understand where filters fit into odor control.

Get Help Choosing and Using Holding Tank Treatment

Not sure which form to buy or how to use treatment in your marine sanitation system? Environmental Marine can help you choose between liquid and packet Max Control Natural treatments and understand how to use them in your sanitation system. You can contact us online, start a live chat, or call 800-522-2656 to speak with one of our marine sanitation experts Monday through Friday, 8:00 a.m. to 4:30 p.m. ET.

Frequently Asked Questions

Can Max Control Natural be used in gray water tanks?
Yes. Dometic identifies Max Control Natural for use in both black and gray water holding tanks. Follow the package instructions for your application. A gray water sump that pumps overboard is different from a holding tank, so check your system before choosing where to add treatment.

Does Max Control Natural contain formaldehyde or bronopol?
No. Dometic identifies the formula as free of both ingredients. It uses enzymes and bacteria to help control odors and break down waste.

Does Max Control Natural replace tank cleaning and pump-out?
No. Treatment helps break down waste between pump-outs, but the tank still needs to be emptied and periodically rinsed to remove accumulated residue.

Why might one flush be insufficient in a VacuFlush system?
The treatment may reach the vacuum generator before reaching the holding tank. We recommend two to three flushes total to help carry it through the system, with the appropriate procedure depending on your installation.

Should I add more treatment if the boat still smells?
Check the dose, delivery method, and odor source first. Treatment left in the plumbing hasn’t reached the tank, and odors from a leaking connection or dirty sump require separate attention. Follow the product’s dosing instructions rather than assuming a larger amount will solve the problem.

Still have questions? Explore our Frequently Asked Questions for expert answers on marine sanitation systems, marine plumbing products, replacement parts, and more.