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Biological Wastewater Treatment

Specialist call 0418414440 Sydney

Biological Wastewater Treatment

This is a brief summary of the various techniques that have been developed to treatwastewater by biological means. They accomplish what is generally calledsecondary

treatment.

Purpose:

The idea behind all biological methods of wastewater treatment is to introduce contactwith bacteria (cells), which feed on the organic materials in the wastewater, thereby

reducing its BOD content. In other words, the purpose of biological treatment is BOD

reduction.Typically, wastewater enters the treatment plant with a BOD higher than 200 mg/L, but

primary settling has already reduced it to about 150 mg/L by the time it enters the

biological component of the system. It needs to exit with a BOD content no higher than

about 20-30 mg/L, so that after dilution in the nearby receiving water body (river, lake),the BOD is less than 2-3 mg/L. Thus, the biological treatment needs to accomplish a 6-

fold decrease in BOD.Principle:Simple bacteria (cells) eat the organic material present in the wastewater. Through their

metabolism, the organic material is transformed into cellular mass, which is no longer in

solution but can be precipitated at the bottom of a settling tank or retained as slime onsolid surfaces or vegetation in the system. The water exiting the system is then much

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clearer than it entered it.

A key factor is the operation of any biological system is an adequate supply of oxygen.

Indeed, cells need not only organic material as food but also oxygen to breathe, just likehumans. Without an adequate supply of oxygen, the biological degradation of the waste

is slowed down, thereby requiring a longer residency time of the water in the system. For

a given flowrate of water to be treated, this translates into a system with a larger volumeand thus taking more space.

Advantages:

Like all biological systems, operation takes place at ambient temperature. There is noneed to heat or cool the water, which saves on energy consumption. Because wastewater

treatment operations take much space, they are located outdoor, and this implies that the

system must be able to operate at seasonally varying temperatures. Cells come in a mix

of many types, and accommodation to a temperature change is simply accomplished byself adaptation of the cell population.

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Similarly, a change in composition of the organic material (due to peoples changing

activities) leads to a spontaneous change in cell population, with the types best suited todigest the new material growing in larger numbers than other cell types.

3Types of equipment for biological treatment:

There are two broad types of biological wastewater treatment: those that include

mechanical means to create contact between wastewater, cells and oxygen, and those than

dont.a) With mechanical means:

1. Activated sludge: This is the most common type. It consists in a set of two basins. In

the first, air is pumped through perforated pipes at the bottom of the basin, air risesthrough the water in the form of many small bubbles. These bubbles accomplish two

things: they provide oxygen form the air to the water and create highly turbulent

conditions that favor intimate contact between cells, the organic material in the water andoxygen. The second basin is a settling tank, where water flow is made to be very quiet so

that the cellular material may be removed by gravitational settling. Some of the cell

material collected at the bottom is captured and fed back into the first basin to seed theprocess. The rest is treated anaerobically (= without oxygen) until it is transformed into a

compost-type material (like soil).

The cost of an activated-sludge system is chiefly due to the energy required to pump air

at high pressure at the bottom of the aerator tank (to overcome the hydrostatic pressure ofthe water). Another disadvantage is that the operation is accomplished in two separate

basins, thereby occupying a substantial amount of real estate.

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42. Trickling filter: A trickling filter consists in a bed of fist-size rocks over which the

wastewater is gently sprayed by a rotating arm. Slime (fungi, algae) develops on the rock

surface, growing by intercepting organic material from the water as it trickles down.Since the water layer passing over the rocks makes thin sheets, there is good contact with

air and cells are effectively oxygenated. Worms and insects living in this ecosystem

also contribute to removal of organic material from the water. The slime periodicallyslides off the rocks and is collected at the bottom of the system, where it is removed.

Water needs to be trickled several times over the rocks before it is sufficiently cleaned.

Multiple spraying also provides a way to keep the biological slimes from drying out in

hours of low-flow conditions (ex. at night).

Plastic nets are gradually replacing rocks in newer versions of this system,

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providing more surface area per volume, thereby reducing the size of the equipment.

3. Biological contactor: This is essentially a variation on the trickling filter, with the

difference being that solid material on which slime grows is brought to the water ratherthan water being brought to it. Rotating disks alternate exposure between air and water.

b) Without mechanical means:The wastewater is made to flow by gravity through a specially constructed wetland.

There, the water is brought into close contact with vegetation (ex. reeds), which acts as a