Biological Wastewater Treatment This is a brief summary of the various techniques that have been developed to treat wastewater by biological means. They accomplish what is generally called secondary treatment. Purpose: The idea behind all biological methods of wastewater treatment is to introduce contact with 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 on solid surfaces or vegetation in the system. The water exiting the system is then much 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 like humans. 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 volume and thus taking more space. Advantages: Like all biological systems, operation takes place at ambient temperature. There is no need 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 by self adaptation of the cell population. 1
Similarly, a change in composition of the organic material (due to people s changing activities) leads to a spontaneous change in cell population, with the types best suited to digest the new material growing in larger numbers than other cell types. 2
Types 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 don t. 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 rises through 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 and oxygen. 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 the process. 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 of the water). Another disadvantage is that the operation is accomplished in two separate basins, thereby occupying a substantial amount of real estate. 3
2. 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 periodically slides 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, 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 rather than 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 4
biological filter to the water. The organic material in the wastewater is used as nutrient by the plants. Oxygen supply is passively accomplished by surface aeration (contact with oxygen of the atmosphere). Since water flow is slow in such system, to give ample time for the biological activity to take place, there is almost no turbulence in the water and reaeration is weak. Compared to mechanical systems, constructed wetlands occupy far more real estate, but they may be aesthetically pleasing, especially if they are well integrated in the local landscape. They emit no odor, but people should stay away because of the danger posed by pathogens. Constructed wetlands have also the least energy requirement. Energy is only needed to pump the wastewater to the entrance of the system, from where gravity and biology do the rest. A major disadvantage, however, is the highly reduced performance during winter, especially in regions where ground freezes during some of the winter months. 5