Introduction to wastewater treatment
Two fundamental terms are used to evaluate the quality of wastewater treatment systems:
influent: wastewater flowing into a treatment plant
effluent: treated wastewater flowing out of any treatment unit (the effluent should be better than the influent as a result of treatment).
Two more terms are fundamental to following the progress through a wastewater treatment facility:
sedimentation: the process used in both primary and secondary wastewater treatment, that takes place when gravity pulls particles to the bottom of a tank (also called settling).
sludge: any solid, semisolid, or liquid waste that settles to the bottom of sedimentation tanks (in wastewater treatment plants or drinking water treatment plants) or septic tanks.
With these terms defined, we can now talk about the first major phase of wastewater treatment, referred to as preliminary treatment.
Preliminary treatment is used to condition the wastewater. Very little reduction in wastes goes on. Instead, it aids the processing that occurs downline. Examples of preliminary treatment include the following.
1. screening- to remove large objects, such as stones or sticks, that could plug the flow through treatment units.
2. grit chamber: a chamber or tank that slows down the flow of wastewater. By doing so, heavy, large solids (grit) settle out and are removed.
Primary treatment is the second major phase of wastewater treatment. It uses physical treatment methods (later on, we will see that secondary methods use biological methods). It removes settleable or floating solids only. When completed, it generally removes less than half of the suspended solids and BOD in the wastewater. By far, the most common method for primary treatment is the primary sedimantation tank (also called the primary settling basin) as shown below. It is a vessel in which solids settle out of water by gravity. The settleable solids are pumped away (as sludge), while oils float to the top and are skimmed off. Sedimentation tanks can also be adapted for secondary and tertiary processes, and can also be used to treat drinking water.
Notice several things about the above graphic. First, the gravitational settling of sludge is sent off to another area for further treatment and disposal. Second, the treated effluent is sent off for further secondary (biological) treatment. And third, this particular design skims off the grease and other floating scum for further treatment and disposal.
The diagram below shows another design. Notice that the "scrapers" move very slowly to help collect the sludge that settles to the bottom.
Primary sedimentation tank
The final diagram given below is the Imhoff tank (not to be confused with the Imhoff cone mentioned above, which is simply glassware used in the lab). Pay special attention to the small opening at the bottom of the smaller chamber. This Imhoff tank is used in smaller municipalities.
Secondary and Tertiary Wastewater Treatment
Secondary treatment uses biological treatment processes. Microorganisms convert nonsettleable solids into settleable solids. Sedimentation typically follows, allowing the settleable solids to settle out. It removes approximately 85% of the BOD and TSS in wastewater. Secondary treatment for municipal wastewater is the minimum level of treatment required by the Clean Water Act. It includes:
1. Activated Sludge- This is the most common option in secondary treatment. It starts with aeration that encourages the growth of microbes in the waste. The microbes feed on the organic material, which then allows solids to settle out. Bacteria-containing "activated sludge" is continually recirculated back to the aeration basin to increase the rate of organic decomposition.
A closer look reveals how air is release from the bottom of the tank.
2. Trickling Filters- These are open tanks of "coarse media" (rocks about 3 inches in diameter -- usually stones or plastic, and not sand!) about 3-10 ft. deep. Wastewater is sprayed into the air (aeration), then allowed to trickle through the coarse media. Despite its name, trickling filters do not actually filter the water. Instead, the larger stones serve as a surface on which microbes grow. These microbes, attached to and growing on the stones, break down organic material in the wastewater. Trickling filters drain at the bottom; the wastewater is collected and then undergoes sedimentation.
3. Lagoons- (oxidation ponds or stabilization ponds): this method uses ponds to treat wastewater. Algae grow within the lagoons and utilize sunlight to produce oxygen, which is in turn used by microorganisms in the lagoon to break down organic material in the wastewater. Wastewater solids settle in the lagoon, resulting in effluent that is relatively well treated, although it does contain algae. These are slow, cheap, and relatively inefficient, but can be used for various types of wastewater. They rely on the interaction of sunlight, algae, microorganisms, and oxygen (sometimes aerated).
After primary and secondary treatment, municipal wastewater is usually disinfected using chlorine (or other disinfecting compounds, or occasionally ozone or ultraviolet light). An increasing number of wastewater facilities also employ tertiary treatment, often using advanced treatment methods.
Tertiary treatment (sometimes called "advanced treatment" ) includes any level of treatment beyond secondary treatment. These processes can be physical, biological, or chemical. Processes for treating drinking water can be adapted to tertiary treatment. They can include: filtration, activated carbon adsorption, and coagulation.
Septic Tanks
A septic tank is a water tight tank. That's it. There really isn't much to it -- wastewater goes in and comes out of it. The key is to think of septic tank systems, which involve more than just the septic tank. In the diagram below, the dirt that would normally cover all of this system is removed for a better view. We see waste moving from the house to the septic tank, and eventually making its way to an "absorption field." Wastewater is draining directly into the soil in the absorption field. In this absorption field, the soil microbes act to digest the wastewater. In other words, the real treatment is happening in the absorption field, not the septic tank. The key is for the wastewater to drain evenly so that the field doesn't turn into mud or, even worse than that, wastewater back up towards the house!
So what does the septic tank do? Taking a closer look below, we see that its fundamental purpose is to slow down the movement of wastes -- typically, it takes about 24 hours for wastes to travel through the septic tank. During this time, some of the solids may settle out and break up, but very little treatment is going on. In the cross section view given below, we see that sludge (with solids) settles to the bottom and scum flots on the top, but basically it is wastewater moving through the tank. However, if the solids have broken up in that 24 hour period, there is less chance for clogging further down the line in the absorption field.
Since the absorption field is where the digestion takes place, we now take a closer look in the cross section diagram given below. The "distribution pipe" shown below has holes in it -- we deliberately want the watewater to drain into the absorption field. We surround the distribution pipe with gravel and cover it with dirt (backfill), but the key is to to drain the wastewater evenly over a wide area.
Backing away from this cross section just a bit, we can see a more active view in the diagram below. Notice that we are deliberately leaking wastewater into the soil. As long as we do this evenly and do not overwhelm the soil with too much wastewater, the soil is able to digest the wastes.
This helps explain some of the concerns with septic tanks:
Objective: The purpose of this section is to define:
and to derive an equation to help predict these conditions.
A cross
connection is
any physical connection between wastewater and potable water, with
the potential for backflow. Backflow is any force pushing the wastewater towards potable
water. In other words, a cross connection can result in contamination
of drinking water, and it is a fundamental concern for environmental
health professionals. In order to understand backflow, we start with
an equation that should be familiar to all physics students:
P =
F/A where P =
pressure, F = force, and A = area
Pressure is typically
measured in psi (pounds per square inch) with two
scales:
psi a
(absolute scale) and psi g (gage scale)
Psig is what we see on pressure gauges, and psia is the larger,
absolute pressure in pounds per square inch. This may seem confusing
at first, so consider two examples:
1. If we open a gauge at sea level (which releases the pressure), it may read zero, but we know that gravity is pushing down at about 14.7 pounds per square inch. Thus, psig = 0, but psia = 14.7
2. If a pump sucks all the air from a closed container (i.e., a perfect vacuum), the gauge will start reading negative numbers. Thus, we know that in a perfect vacuum at sea level, psia = 0 (by definition), but the gauge will read psig = -14.7 In each example, psig = (psia - 14.7).
Using these units, we can now define backsiphonage as backflow that occurs in a negative psig (i.e., backflow caused by sucking liquids). In contrast, backpressure occurs in a positive psig (i.e., backflow caused by a higher pressure fluid forcing itself against a weaker pressure fluid).
We can study
backsiphonage by deriving an equation for the pressure of a water
column at any height. Consider the pressure for one cubic foot of
water. Since once cubic foot of water weighs 62.4 pounds, and this
cube of water rests on an area that is 1 sq. foot, we can calculate
the psi reading with the equation P = F/A:
P = F/A = 62.4
lbs. / 1 sq. ft. = 62.4 lbs. / 144 sq. in. = .433 psi
Next, for two cubic
feet (one on top of the other), we double the weight resting on the
same square foot:
P = (62.4 lbs.
/ 1 sq. ft.) * 2 = .866 psi
Finally, for any
number of cubic feet:
P = (62.4
lbs/sq. ft.) * H (where H is the height in feet)
To generalize this equation for any water column (Pw):
1) divide 62.4 lbs. by 144 square inches (62.4 / 144 = .433),
2) add any other existing pressure in the system (P), and we then get the general equation: Pw = (.433*H) + P
VARIABLES:
|
P = pressure (psi) |
psig = lbs./square inch (gage) |
|
F = force (lbs.) |
water = 62.4 lbs./cubic ft. |
|
A = area (square inches) |
Pw = pressure in a water column (psi) |
|
psia = lbs./square inch (absolute) |
H = height of water column (in ft.) |
PROBLEMS
1. If a 100' building has 20 psi at the top floor, what is the pressure at the bottom?
|
H = 100' |
|
P = top = 20 psi |
|
Pw = ? |
2. If the pressure drops by 30 psi, what are the new pressures at the top and bottom floor? (hint: the pressure drops by exactly 30 psi throughout the system)
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