Terminology Used in Waste Water Treatment

The terminology used in the treatment of waste water is often confusing. Terms such as primary, secondary and tertiary treatment, in the treatment of municipal waste water, frequently appear in the literature, and their usage is not always consistent.

The meanings of these terms, as used in Chapters 5 and 6 are therefore outlined in this section. Figure 2.1 shows a flow diagram of a typical sewage treatment plant, and indicates the different nitrogen removal steps.

The latter part of this section will show at which step the removal of the nitrogen compounds in the waste water takes place.

Primary treatment:

Primary treatment removes solid material from the incoming waste water. Large particles are removed by screens or reduced in size by grinding devices. Inorganic solids are removed in grit channels and much of the organic suspended solids is removed by sedimentation.

A typical primary treatment system is shown in Fig. 2.2. The primary treatment system will remove almost one-half of the suspended solids in the incoming waste water.

The waste water transported to secondary treatment is called the primary effluent.

Secondary treatment.

Secondary treatment usually consists of a biological conversion of dissolved and colloidal organic compounds into biomass, and its respiration. Some nutrient removal takes place in secondary treatment units, depending on the ratio of heterotrophs and nitrifier in the different unit processes. The different unit processes during secondary treatment are the so-called combined carbon oxidation and nitrification processes. Fig. 2.3 shows the secondary treatment process.

Secondary systems normally produce an excess biomass that is sometimes recycled into the secondary treatment with the influent.

Primary and secondary treatment can sometimes be accomplished simultaneously in an oxidation pond or an aerated lagoon, as shown in Fig. 2.4.

In an oxidation pond, the oxygen is supplied from natural sources, and the oxygen concentration, is therefore low, that is why oxygen rarely penetrates to the bottom of the pond, and the solids that settle are decomposed anaerobically. In aerated lagoon systems, oxygen is supplied by mechanical aeration, and the lagoon is, therefore, aerobic.



Typical Sewage System Diagram
Figure 2.1 Flow diagram of a typical sewage treatment plant.



Figure 2.2 Plan of a primary treatment process.

Effluent recycle j

Effluent recycle j

y To sludge treatment/ sludge return

Figure 2.3 Plan of a secondary treatment process.

Tertiary treatment.

The reliability of stable processes has become increasingly important in order to meet today's effluent standards for the nitrogen content in a waste water. It is, therefore, often necessary to introduce another treatment step to refine the waste water. Tertiary nitrifying or denitrifying steps are normally the same processes as described under secondary treatment; but the concentration of a nitrifying or denitrifying biomass is much higher, because the influent of organic compounds into a tertiary nitrifying treatment is so low, that it will not cause a competition between the heterotrophic and nitrifying bacteria, and thus lowers the nitrification rate. Tertiary nitrifying unit processes have, therefore, a higher nitrification rate than the combined oxidation and nitrification steps.

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