How Do RV Black & Grey Water Tanks Work? (Explained)

With an RV, you’ll typically store your waste and periodically dump it out at a dump station.  The most common way of doing this is with a black water tank.

How does an RV black water tank work?  An RV black water tank has plumbing that runs into a tank.  This tank is usually located underneath of the RV and it needs to be drained before it becomes too full. You empty the black water tank in a dump station.

How Often Should You Empty a Black Water Tank?

A black water tank should be emptied either before it becomes full or before you go on a trip with it.  While it is OK to travel with some waste in your black water tank, it is not optimal.  A full black water tank adds weight to your RV and lowers its gas mileage.

For instance, a full 30-gallon black water tank will weigh over 240 pounds.

A full black water tank also moves the tank around and can cause the tank and the surrounding areas to age prematurely.  For example, the part of the tank that is bolted to the frame may wear out more quickly due to the additional friction caused by the movement of the waste inside of the black water tank.

Tanks will usually have a gauge in them to let you know when they are close to full.  This gauge can be a manual gauge that must be visually inspected from time to time, or it can be an automatic gauge that puts off an alert when your tank becomes too full.  Gauges can give false readings when the tank has an obstruction within it.

One thought to keep in mind is that there is such a thing as dumping your black water tank too frequently. 

If you empty your tank too frequently, the solids will not have a chance to break down.  This means that you’re more likely to clog your hoses when emptying your tank out.  Many RVers have found that it is best to empty your tank only after it is about 70% full.

How Big Is a Black Water Tank?

Blackwater tanks vary dramatically in size.  Asking how big a black water tank is, is like asking how long a string is.  The answer of course is, it depends!

In general, the larger the RV, the larger the black water tank.  However, this isn’t always true so you’ll want to check the size before you buy.

Here are a few examples for you to review:

The Airstream 23′ Serenity FB –  This RV has an 18-gallon black water tank.  It sleeps 4 people and has a 21-gallon gray water tank.  With 4 people, an 18-gallon black water tank will fill up quickly.

The Wilderness 2500 RL by Heartland – This camper has a 40-gallon black water tank and a 40-gallon gray water tank.  It is almost 32 feet long and it comfortably sleeps, 5 people.  The size of this black water tank is large and the owner might be able to go two weeks before having to empty it.

The Terry Classic V21 by Heartland – This camper has a 27-gallon black water tank and a 44-gallon gray water tank.  It is 22 feet long and sleeps, 4 people.  The tank is an appropriate size for a 22-foot long camper and the owner could probably go a week before having to empty the tank.

As you can see, gray and black water tanks are not always the same size.  A camper will usually have a larger gray water tank than a black water tank.  The reason for this is that gray water tanks tend to fill up quicker than black water tanks.  Showering and washing dishes takes a lot of water and this must all go into the gray water tank.

How to Empty a Black Water Tank

A black water tank should never be dumped into a storm drain or out into a field.  This wastewater is harmful to people, animals, and the environment.  A black water tank should only be emptied out into designated black water tanks.  You can find these at RV parks, state parks, national parks, and some rest areas.

To empty your tank, you’ll need to first locate the exterior valve.  Once you’ve found it, connect one end of your RV sewer hose to your RV and the other end to the dump station.

Here’s a guide to using Dump Stations.

It is always best to wear rubber gloves and old shoes during this process as it is possible to get waste water on your hands and feet.  Some people take it a step further and wear old clothes to the dump station.

Once everything is connected, open the black water tank valve and drain the tank.  After everything is empty, you’ll connect a hose to your black tank flush valve.  Run this until you see clear water run out of your sewer hose.  Close the black water tank valve when this is complete.

Now your black tank is empty and you can empty your gray water tank.

Emptying this tank out second will help clear out any wastewater that was left behind after emptying the black water tank. Close the gray water tank valve when this is done and carefully disconnect your sewer hose from the RV.  Slowly lift the sewer hose and let any remaining water run into the dump station.

When this is complete, you can disconnect the sewer hose from the dump station.

Clean off the hose and the connections and spray everything with disinfectant.  When everything is clean, you can put it all back into your RV and get back to camping.  Just be sure to remember to put your sewer cap back on before you head out.

How to Clean a Black Water Tank

Cleaning out the black water tank will help prolong its life.  It will also help control odors and it will ensure the accuracy of the fill gauge.

If the RV has a built-in system for flushing the tank, this is an easy process.

You’ll just connect a hose to the flush valve and run water through the tank as we discussed in the last section.  If the RV does not have a system built into it, you’ll need to do this manually.

This can be done with a special RV flush valve or a tank wand.  The RV flush valve will connect from the outside.  It shoots water back into the tank to help clean it out. A tank wand is connected to a hose and is inserted into the toilet.  It sprays water down into the black water tank and helps clear out any waste that has built up within the holding tank.

Tank wands can be solid pipes with valves at the end or they can be flexible with built-in spray holes at the end.  Both types of tank wands are easy to use and inexpensive to buy.

Special soaps and deodorizers are rarely needed to clean a black water tank.  However, some people do dump a mixture of bleach and water into the tank when cleaning it.  Caution should be used when employing bleach to clean out your tank as bleach can eventually break down plastic tanks.  When using bleach, always make sure you thoroughly rinse out your tanks afterward.

How to Have a Bathroom Without a Black Water Tank

Many smaller RVs and vintage RVs do not come with black tanks or bathrooms at all.  In this case, you can add a black tank or you can use one of the many alternatives.  Some people prefer to use an alternative solution rather than have to deal with emptying, cleaning, and maintaining a black water tank.

Cassette Toilets

A permanent solution to eliminating the black water tank is to install a cassette toilet.  These toilets often look just like a standard RV toilet and they can come with both manual flush systems and electronic ones.  The toilet is permanently fixed into the RV and the tank is built into the bottom of it.  This tank is small and can be pulled right out of the RV from the outside.  It can then be dumped at a dump station or into a regular toilet at home or in a rest station.

Cassette toilets are usually more expensive than standard toilets and cost around $500.00 to buy.

Examples of Cassette Toilets

The Dometic CTS4110 – This toilet is lightweight and is great for vans and lightweight RVs.  It has an electric flush and holds 5 gallons of wastewater.  Wastewater is removed through an exterior service hatch.  The cost of this toilet is $599.00.

The Thetford 32812 – This toilet has an electric button flush, a 5-gallon holding tank and an LED display that indicates when the waste tank is full.  The holding tank is on wheels and can be removed from outside. The cost of this toilet is $515.00.

The Thetford Permanent Rotating Toilet – This toilet can rotate at a 180-degree angle.  It has a 4.75-gallon holding tank and an electric flush.  The cost of this toilet is $550.00

All of these toilets store their own freshwater for flushing, so you don’t have to worry about adding any plumbing to use this type of bathroom.

Portable Toilets

Portable toilets are very similar to cassette toilets and often the name cassette toilet is used to describe these as well.  The main difference is that this type of toilet is not permanently attached to the RV.  It will not have a tank that is accessible from the outside, but it will cost dramatically less money to purchase.  You can expect to pay anywhere from $50.00 to $300.00 for this type of toilet.

Examples of Portable Toilets

The Camco Standard Travel Toilet – This toilet comes in two different sizes.  It can be purchased with a 2.6-gallon tank and a 5.3-gallon tank.  The tank sits below the toilet, so the smaller the tank, the lower the toilet.  This toilet has a manual flush system and sells for $50.00 or $75.00 depending on the size of the tank.

The Thetford Porta Potti 260B – This toilet has a 2.6-gallon waste water tank and a manual flush.  It sells for around $100.00.

The Palm Springs Outdoor Camping Toilet – This toilet has a 5-gallon waste water tank and a 3-gallon freshwater tank.  It sells for $65.00.

Composting Toilets

A composting toilet does not make use of water at all.  Instead, waste is sent into a holding device and odors are controlled through the use of exhaust vents and drying materials.  These toilets can be very high tech or very low tech.  They can be made at home with just a bucket and a toilet seat or purchased for well over $500.00.  The more expensive models will have built-in exhaust vents and fans and will look more like standard toilets.

Popular drying materials for composting toilets are sawdust and peat moss.  Some people also use soil as both a drying and covering agent.

Once the toilet’s holding tank is full of waste and drying material, it is put into a composting pile.  Unfortunately, it is not legal to create a human-waste compost pile in certain areas of the country.  You’ll want to check your local laws before buying this type of toilet.

Examples of Composting Toilets

Nature’s Head Dry Composting Toilet – This toilet comes with a 12-volt power plug and a 5-foot vent hose.  It also has a crank handle to rotate waste so that it decomposes faster.  A Nature’s Head toilet runs in the neighborhood of $1,000.00.

Sun-Mar Composting Toilet – This toilet is large and heavy and is better suited to tiny houses and large RVs.  It currently sells for about $2,000.00.

Incinerating Toilets

An incinerating toilet is a toilet that burns your waste.  It typically uses either an electric power source or a propane power source.  The toilet can take up to an hour to run but will burn several days of waste into a small pile of dust about the size of a tablespoon.  This dust can then be thrown out with the garbage.

This type of toilet needs to be properly vented and may not be good to use in an RV that will be frequently moved.

Incinerating Toilet Examples

The Incinolet Model CF – This toilet is an electric incinerating toilet that uses 2,000 watts of energy and runs off of 120 volts.  It sells for $1,900.00.

The EcoJohn SR12 – This toilet can be fueled with propane, natural gas, diesel, or electricity.  It is a large toilet that can accommodate the needs of over 10 people.  The cost of this toilet is $4,000.00.

Dry Flush Toilets

A dry flush toilet is a specialized toilet that uses canisters and bags to compress waste into a holding tank.  It is great for inhibiting odors and can store up to 15 flushes.  The bags are moved and compressed via an electric motor that runs off of a 12-volt power source.  The advantage of this toilet is that you do not need any water to operate it and you do not have to worry about composting the waste afterward.

The downside to this toilet is that you must buy special bags to operate it.  Also, it may be illegal for you to throw out human waste in your area.  For example, in some communities, people aren’t even allowed to directly throw baby diapers into the trash.  Parents or caretakers are expected to first dump the feces into the toilet before throwing the diaper away.

The reason for this is that communities do not want human feces in their landfills as it can be a danger to the community.

Dry Flush Toilet Example

The Laveo – Dry-flush is currently the only maker of this type of toilet.  They have an AC powered model and a battery-powered model.  The battery is a 12-volt battery and it is contained within the toilet.  Both of these toilets sell for the same price of $595.00.

Waste Bags

One of the most low-tech methods of getting rid of waste without a black water tank is to simply use a trash bag.  Some people buy folding toilets that have bags attached to them and others attach a toilet seat to a bucket and place the bag inside of that.  Biodegradable bags can be purchased so that the waste can be buried or people can simply use standard trash bags.

The big advantage to this setup is that it is extremely inexpensive and easy to use.  The downside is that the bags will need to be stored and there can be odor issues related to this type of system.

To reduce odor issues, some people will line their bags with cat litter or some other substance that is meant to quickly dry out human waste.

People who choose to go this route will need to make sure they are allowed to throw their bags out or legally bury them.

Examples of Toilets Used With Waste Bags

Green Elephant Folding Commode Portable Toilet Seat – This toilet seat is just a seat mounted onto a set of folding legs.  If you’ve never seen one, imagine a small folding camp table with a toilet seat on it instead of a table-top.  A bucket with a bag inside can be placed underneath of it.  This sells for $40.00.

ShineLife Portable Folding Toilet – This toilet is made from plastic and fabric.  It folds up and does not need a bucket, but a bag will need to be placed inside of it.  This toilet also sells for $40.00.

What Is the Difference Between Black and Gray Water

Blackwater consists of human waste.  It is the water that comes from an RV’s toilet.  Gray water, on the other hand, comes from kitchen and bathroom sinks.  A gray water tank can sometimes be emptied out onto the ground in certain parks.

Some homeowners will empty their gray water tanks into their yards.  This being said, never empty a gray water tank out into a storm drain.

While gray water is not as toxic as black water, it can still cause damage to the environment and it will pollute local water sources. 

Dumping this water into a grassy area is much healthier for the environment as the plant life and the soil will have a chance to filter out the gray water before it reaches local streams, rivers, lakes, and oceans.

Some RV manufacturers only have one tank that serves as both a black water tank and a gray water tank.  This tank must always be treated as if it were a black water tank.  An example of an RV that combines the gray and black water tank is the Airstream Bambi.  This 16′ camper saves room by combining the two tanks.

How to Empty a Gray Water Tank

A gray water tank can be emptied the same way that a black water tank is emptied.  The gray water tank is emptied into a dumping station after the black water has been dumped.  This helps to reduce the amount of cleaning needed on the sewer hose as the gray water will force the rest of the water from the black water tank through to the dump’s holding station.

How to Clean a Gray Water Tank

Gray water tanks do not need to be cleaned the way a black water tank does.  This is because you are not dumping any solid waste down a gray water tank.

If you do end up with an obstruction in your gray water tank, you can use a flexible tank wand to jet the line.

Gray Water Tank Alternatives

Some RVs were created as dry RVs.  These types of RVs do not have any holding tanks and a gray water tank must be installed or alternative methods must be used.  There are a couple of different alternatives to choose from.

Portable Gray Water Tanks

One alternative to a standard gray water tank is a portable gray water tank.  Portable gray water tanks do not have to be mounted and they can be taken to a dump station without moving the actual RV.  A portable gray water tank can range anywhere from 10 gallons to over 40 gallons.  Prices will vary based mostly off of the size of the portable tank.

Examples of Gray Water Tanks

Barker Manufacturing Company 16 Gallon Tank – This tank is 16 gallons and sits on four wheels.  It has a round handle so that it can be pulled or placed over a tow vehicle’s ball hitch.  There is an inlet to collect water on top and a hose on the side to be used at the dump station.  The tank can be purchased for $200.00.

The Thetford SmartTote2 – This tank holds 27 gallons of gray water and has four wheels as well as a handle that can be pulled by hand or on the back of a tow vehicle’s ball hitch.  It is especially important that larger tanks like this are able to be towed as they can weigh over 300 pounds when full.  These tanks sell for just over $200.00.

The Camco 39000 Rhino – This tank holds 15 gallons of gray water and has two wheels on the back.  The wheels are large and the front handle can be pulled or towed.  There is an inlet hole for gray water as well as an inlet hole to attach a hose for cleaning.  Underneath the hose’s inlet hole is a built-in spinning sprayer.  This helps to clean the interior and to remove any debris that may have built up inside the tank.

A 3-foot sewer hose, two elbows, a water hose, and a ladder hook are all included with the water tank.  This tank sells for around $150.00.

Water Jugs or Buckets

Another option for replacing gray water tanks is to just use water jugs and buckets.  This method is especially good for those that do not have any plumbing built into the RV.  Water jugs can be used to provide clean water and buckets can be used to catch gray water.

The advantage to a setup like this is that the jugs and buckets will never get too heavy.  Remember a gallon of water weighs over 8 pounds.  This means that even a small 10-gallon portable gray water tank will weigh over 80 pounds.

Five-gallon buckets can be purchased from big box stores like Home Depot and Lowes for less than $10.00 dollars.  Jugs of water can easily be purchased just about anywhere for less than $20.00.

In Summary

A black water tank works by first collecting wastewater and then storing it until it can be dumped at a dump station.  This water is bad for people, pets, and the environment and must always be dumped at a proper dump station.

Blackwater tanks must also be cleaned, maintained, and replaced periodically.

Gray water tanks serve the same purpose except they store water from sinks rather than human waste.  These tanks usually need to be dumped at a dump station, but can sometimes be dumped onto a lawn, field, or other areas that have soil and plant life to filter the water naturally.