Wastewater Disposal

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At the end of the water supply pipes in your home are fixtures or appliances where the water is used. The sys tem that carries the used water away is the wastewater drainage system, or, as it's called in the trade, the DWV or drain-waste-vent system. Because of the nature of all the household and body wastes that are put into our used water, we need more than just drain pipes to take this water away. Our waste materials contain large amounts of bacteria, both beneficial and harmful varieties. The anaerobic bacteria, those which live in an environment without free oxygen, thrive in our digestive tracts. When these bacteria are in septic tanks, and sewer lines, they produce a foul-smelling, poisonous, and flammable mixture of gases, commonly called sewer gas. This gas must be kept out of our homes.

To prevent sewer gas, pests such as rats, and other contaminants from coming up the drain pipes into our homes, we add traps to the system. A trap is a U-shaped curve in a pipe or fitting that remains filled with water at all times. Sewer gas and other things can't get past the water and come up through the drains. Each time you empty a sink or flush a toilet, the water in the trap is also flushed away and new water replaces it. This means, theoretically, that dirty water isn’t in the trap long enough to grow large colonies of its own bacteria.

To get rid of the sewer gas and to prevent siphoning of water out of the traps, all house drain systems have vents. Vent pipes come off the drains downstream from the traps and go up through the roof. Sewer gas passes up the vent pipes and is dispersed harmlessly into the air. When water flows down the drain, air is sucked down the vents into the pipes, equalizing the air pressure on each side of the trap. This prevents the water in the trap from being siphoned out and sewer ,gas from entering the house.

Also, since all drain systems are subject to clogging from time to time, they are provided with cleanouts. These are Y or T fittings with screw-on covers that give the home owner or plumber a place to insert rods or augers for the purpose of dislodging clogs.

Drain pipes. Drain pipes differ from water supply pipes in at least two important ways. They are larger and they flow at a lower pressure, usually by gravity alone, The smallest drain pipe in the house, probably the one coming from the bathroom sink, is never smaller than 1¼- inch and 1 1/2-inch is usually preferred by the building code. The smallest drain from a toilet is 3 inches in diameter. Metal drain pipe 2 inches or less in diameter is usually made of galvanized wrought iron, galvanized steel, or copper tubing. In larger sizes, the pipe is made of cast iron. Most building codes now allow plastic pipe for DWV systems in single-family homes, but there are usually some restrictions.

The DWV system in a house is arranged like a tree, with the smallest branches near the top. The smallest pipes always flow into pipes the same size or larger until they all flow into the soil stack, building drain, and building sewer, which are the largest pipes in the system. The sizes of all these pipes are specified in the building code. A typical arrangement of pipes with their usual sizes and names is shown in the drawing.

If you are going to replace a section of pipe or re place an old fixture with a new one, use the same sizes of pipe, trap, and vent that were there before. If you are putting on an addition that includes a new bathroom or laundry, you should check with the code to be sure of the required pipe sizes. Unlike some specifications in the building code, pipe sizes aren't minimum sizes. The theory of pipe sizing prescribes sizes that are neither too small nor too large, but just right. Pipe that's too small won’t allow the proper amount of material through and may clog or unduly retard the discharge of wastewater.

Not so obviously, horizontal drain pipe that's too large is also undesirable. Larger pipe results in slower movement of the waste material through the pipe. This slower speed reduces scouring and lets heavier particles settle. The greater thermal mass of the larger pipe also causes greater cooling and solidifying of grease. All these things contribute to the possibility of clogging. Unnecessarily large pipe also adds unnecessarily to the cost, whether the pipe is horizontal or vertical.

ill.14 Detail of Part of the DWV System: Branch drain to laundry— 1½- or 2-inch pipe; Lavatory p-trap— 1½-inch pipe; Revent for lavatory— 1½-inch minimum; should be the same size as the branch drain. Lavatory branch drain— 1½-or 2-inch pipe; Main soil stack— 3- or 4-inch hub-and-spigot pipe; Closet bend for toilet— 3- or 4-inch; Branch drain to kitchen sink—1.5 or 2-inch pipe; House drain— same size as the main stack cleanout

The proper size pipe allows for enough capacity to prevent back-ups, to promote the scouring action of swift movement, and to avoid siphoning or blow-back by allowing enough vent air. For drain pipe it's recommended that you always use the smallest size permitted by the code.

Besides making the drain pipe smaller, there is another way to increase the velocity of the waste within the pipe. That is to increase the pitch or slope of the horizontal parts of the system. Codes require that horizontal branches slope ¼ inch per foot. In practical application, this slope varies from 1/16 to ½ inch per foot, and of course the more the pitch, the faster the water will flow. If you increase the slope of a fixture drain pipe, be careful not to make the outlet from an unvented length of pipe lower than the bottom of the fixture trap it serves. If it's lower, it's likely to siphon the water from the trap (see drawing).

Vent system. Building codes require a vent system that's arranged and sized to provide the best possible pres sure/suction relief for each fixture in the system. Systems that don’t follow the code may have back pressure problems such as sinks that drain too slowly, toilets that need several flushes to get rid of all their contents, and blow- back through first floor fixture traps. A poor vent system can also provide too much “negative pressure” (suction), which siphons the water from traps, or “positive pressure”, which forces bubbles of sewer gas through the liquid in the traps (blow-back). Either way the smell isn't desirable.

The size of vent pipe can't be smaller than required by the code. The sizes are based on the kind of fixture, the diameter of the drain being vented, and the length of the vent pipe. For each dwelling unit, most codes require a 3-inch or 4-inch main stack extending through and above the roof. The diameter of an individual vent can never be less than 1¼ inches or less than one-half the diameter of the drain it serves, whichever is larger.

Traps. Building codes everywhere require that each fixture connected to the household drain system have a water-seal trap. Some fixtures, toilets for example, have their trap built in as an integral part of the fixture. The trap most commonly used in the home is called a P-trap because it looks like the letter “F” It is used on sinks, laundry trays, and most other fixtures that don’t have built-in traps.

Building codes have many restrictions on traps because they are so important in protecting the health and welfare of the home’s inhabitants. The following restrictions appear in virtually every code:

• Traps must be self-cleaning. That is, they must be smooth inside so hair, lint, and other material can't be caught and retained.

• No trap can depend on moving parts for its seal.

• No trap outlet can be larger than the fixture drain it's serving.

• Each trap must have a water seal no less than 2 inches or more than 4 inches.

• All traps must be installed level in relation to their water seals to prevent siphoning.

• Each plumbing fixture must have its own trap. There are some exceptions whereby installations of sinks and laundry trays, when there are two or three units adjacent to each other, can be connected to a single trap.

• No fixture can be double trapped. A toilet with an integral trap, for instance, can't be connected to another trap.

Ill. 45

Trap Warning: Water seal level, Tailpiece, Waste arm, Drain pipe stuboff. If the drain stub and waste arm aren't set level, water running through the trap can siphon the water from the trap. A trap that isn't level also makes for a bad joint at the tailpiece.

Vent Alternative: Instead of a secondary stack or a re-vent for a sink or tub, you can use an automatic vent valve. It can't be sealed in a wall but must be accessible for inspection. Tailpiece, Trap

When no water is running down the drain, a spring holds the rubber diaphragm snugly against its seal, keeping sewer gas safely in the pipe.

When water runs down the drain, it creates negative pressure In the vent pipe. The air pressure outside pushes the rubber diaphragm open and lets air in.

Ill.16 Typical P-Trap Assembly – Washer, Compression nut, Washer, Compression nut

Again, if you are replacing a fixture or adding fixtures similar to those already in your house, use a trap like those already in use and you will most likely be well within the requirements of the code. If you are installing a new fixture unlike anything you already have, consult a plumber or your building inspector for advice on the local requirements.

Cleanouts. All building codes now recognize the importance of accessible cleanouts and require them on all installations. The requirements regarding location, size, and minimum distance between cleanouts are spelled out very specifically. For example:

• A cleanout is required where the building sewer crosses the property line or connects to the public sewer. This cleanout allows for the cleaning of stoppages that may occur in the public sewer lateral and also is the place where tests can be performed on the entire house system.

• Some codes require and some allow a cleanout out side the house within five feet of the building.

• Accessible cleanouts are required on all horizontal drain lines and where there is a change in direction of more than 45 degrees in the building drain.

• A cleanout is required at the base of all stacks.

• All cleanouts must have at least 18 inches of clearances to allow the access of cleaning rods, snakes, and other tools.

Sometimes in one-story buildings, the local code will consider a roof vent to be a cleanout for the stack it serves. Certain requirements involving pipe sizes and changes in direction must be met for this to be the case, however. Be sure to check your local code.

Drainage from Below Sewer Level

Sometimes it's necessary to dispose of wastewater that accumulates below the main sewer line. Homeowners will occasionally want to install wash trays, an automatic washing machine, or even a whole bathroom including a sink and a toilet in a basement area. When ground water seepage or rainwater runoff repeatedly floods a basement, facilities must be installed to automatically remove the water. Discharge of rainwater into a sanitary sewer isn't permitted in some areas.

Sump pumps. Groundwater, rainwater, or gray water— water from a washing machine, bathroom sink, shower, or bathtub—can be handled very well with a sump pump. The wastewater is made to run into a concrete-lined sump pit and is then lifted into the sewer with a pump that starts automatically when water reaches a certain level in the pit.

If seepage or rainwater is flooding the basement, the sump pit should be located at the lowest point in the basement. A sump pit for a sink or washer can be any where that it’s convenient to run a drain pipe. The size of the pit will be determined by the size and kind of pump you install.

The oldest, most common, and least expensive sump pump is the upright type. It consists of an electric motor on top of a pedestal. The base of the pedestal, containing the pump and discharge pipe, rests on the bottom of the sump pit. A ball float in the pit is connected by a rod to the motor switch. When the water in the pit reaches a predetermined level, the float flips the switch to ON. When the pump lowers the water level, the ball descends and turns the switch to OFF. The water level to turn the switch off is usually set at about 6 inches because the pump can be damaged if it empties the pit and runs dry. The pit for an upright sump pump should be 12 to 24 inches deep and 12 inches or more across; it can be either round or square.

The submersible pump is more expensive, but generally more satisfactory than the upright type. The submersible pump can't be damaged by flooding and requires much less maintenance. It can run longer and safely take the water level lower than the upright pump. Two kinds of switch mechanisms are available on these pumps. One is the float type similar to the one described on the upright pump. These are subject to jamming if dirt or other debris accumulates in the sump pit. The more desirable type of switch is activated by water pres sure. This one, too, can be set to go on and off at specific depths. The sump pit for a submersible pump should also be at least 12 inches across and from 12 to 15 inches deep.

The installation of a sump pump is a relatively simple project. The first step is to prepare the pit, which should have a level concrete bottom and sides of either concrete or a section of concrete or terra cotta pipe 12 inches or more in diameter.

Break a hole in the basement floor and dig a hole a little wider and 3 or 4 inches deeper than the pit will be. Put 3 or 4 inches of gravel in the bottom and set a wooden form or pipe section into the hole.

Pour concrete in the bottom of the hole and around the pipe or form. Smooth and level the bottom and smooth the concrete around the edge even with the basement floor.

When the concrete is set, put the pump in place and make all the necessary pipe connections leading into the sewer line or seepage pit. A check valve and union are desirable, in fact mandatory in many codes. Before you plug in the pump, be absolutely sure that the receptacle is grounded.

You can make a cover for the sump pit of 0.75” exterior plywood. For the upright pump, three holes will have to pierce the cover—one for the pedestal, one for the float ball rod, and one for the discharge pipe. For the submersible pump, only a hole for the discharge pipe will be needed. Drill the hole(s) and then cut the cover in half through the hole(s). Put the pieces together over the pit and hold them together with straps of aluminum or wood held with screws.

Up-flushing toilets. In some parts of the United States, mostly in the Northeast, single-unit up-flushing toilets are available and allowed by building and health codes. These toilets usually depend on high water pressure, 40 psi or more, to break up the solids into small pieces and siphon them upward to an overhead drain line. Up-flushing toilets are fairly easy to install yourself. They usually come with complete, illustrated instructions that must be followed exactly.

Another alternative that's more universally avail able is a sewage ejector, which is a tank with a pump inside. Material from a regular toilet is flushed into the tank and then pumped up, as much as ten feet or so, to the sewer line. You may find that it’s more convenient to walk upstairs to go to the bathroom than to pay for this installation, however. The cost of a toilet, the ejector, and all the pipe and fittings will run you around $900 or so, even if you do all the work yourself. Having someone install it could increase the cost to $2,000 or more.

The sewage ejector tank is about 30 inches high and 20 inches in diameter, A 4-inch inlet pipe enters the side of the tank about 20 inches below the top, and a 4-inch discharge pipe comes out the top to be connected to the sewer line overhead. Since the ejector tank inlet must be below the toilet discharge, there is no way of installing one of these without excavating below the basement floor (see drawing),

Ill.17 Pedestal Sump Pump: 1.25” discharge pipe, Plywood cover

Sewage Ejector: Standard toilet

Submersible Sump Pump: Submersible sump pump with built-in float switch, Terra cotta drain pipe, Backfill (dirt), Gravel


Saturday, December 10, 2011 9:45 PST