Newer Approaches to Sewage Treatment

2.12. Newer Approaches to Sewage Treatment

In actual practice, the urbanized countries in the world discharge an enormous volume of indus- trial and domestic sewage which need to treated biologically so that the treated sewage could be used for irrigation purposes usefully and effectively or may be discharged to certain extent into a running stream of water (river, rivulet etc.).

The latest methods available for the adequate sewage treament are of three major types, namely : (a) Classical aerobic activated sludge process either with surface aeration or with oxygen intro-

duced by forced compressed air, (b) Aerobic treatment process, and (c) Anaerobic digestion process.

All the above three methodologies shall only be discussed briefly in the sections that follows :

2.12.1. Classical Aerobic Activated Sludge Process

Fig. 6.3 vividly illustrates the classical aerobic activated sludge process with surface aeration or one may even introduce forced compressed air to cause the desired aeration. The main purpose of injecting air in the on-going aerobic process is to minimize drastically the BOD* and COD** of the untreated sewage material to make it fit for disposal either for irrigation utilities or gradual disposal in a current of fresh running water. The sedimentation basin as shown in Fig. 6.3 are two in number, the first — is called the primary sedimentation basin ; whereas the second — is termed as the secondary sedi-

mentation basin. The sludge obtained from the second sedimentation basin is recycled into the raw sewage as well as the aeration basin to augment the activation of the sludge to lower the COD and BOD of the sewage material being introduced initially.

Raw basin

basin

sewage

Aeration basin

Exit

Sludge recycle

Aeration

Sewage

(a) : Aeration Basin

(S)

(b) : Oxidation Ditch

S = Sedimentation basin

Exit

Fig. 6.3. Classical Aerobic Activated Sludge Process.

* BOD : Biological oxygen demand. ** COD : Chemical oxygen demand.

PHARMACEUTICAL BIOTECHNOLOGY

Disadvantages : The various disadvantages of the aerobic systems are as follows : (1) Being an open construction the effective process control is restricted enormously, (2) No measure to control the growth of uncontrolled population of organisms, and (3) Gives rise to severe environmental pollution caused due to odour emission, and fog

formation. Fig. 6.4 depicts explicitely the process which evidently uses the anaerobic digestion. In this

particular instance, the biological process either commences occasionally directly up the raw sewage or rather more frequently upon the solid material (i.e., sludge) usually recovered from sedimentation from the ‘aerobic sedimentation process’ described earlier (see Fig. 6.3).

Gas removal

e cl cy

Re

Fig. 6.4. Diagramatic Sketch of a Sludge Digester.

It is a stark reality that usual developments that took place in ‘sewage treatment’ operations during the past century or so have been accomplished primarily and predominantly in an absolute em- pirical manner. Since the advanced aerobic and anaerobic sewage treatment processes are being studied on a more scientific footing, it is felt overwhelmingly that optimization shall certainly lead to the ultimate maximized success in the positive improvement in these newer processes. In fact, the earlier elaborative investigations were solely and primarily emphasized with regard to an in-depth understanding of the C- metabolism ; however, the futuristic approach shall be focused and anticipated exclusively at a rather more divergent field including S, N and phosphate metabolism, because ultimately the complete elimi- nation of these elements from ‘treated sewage’ shall be the ‘prime statutory demand and require- ment’ as well. Besides, strategic and critical accumulations of heavy metals (e.g., Pb 2+ , Hg 2+ , Ag + , Bi 2+ ,

Cu 2+ , As 3+ , Sb 3+ , Sn 4+ ) in the stabilized sludge are being increasingly recognized as a matter of serious concern. Perhaps ‘bioextractive metallurgy’ (see section 6.2.5) may even prove to be an effective meas- ure in this process also.

Bulking : Another vital, critical, and important problem is the prevailing phenomenon of bulking

in sludge, that essentially prevents and checks the ensuing settling process particularly taking place in the ‘clarification basins’. It has been duly observed and revealed that bulking is prevalently caused due to the development of filamentous microorganisms which fail to settle down adequately and

ADVENT OF BIOTECHNOLOGY

Bulking may be controlled effectively in certain specific instances by adopting the following means and ways :

(a) Introduction of ‘floatation processes’ — in domestic sewage treatment and disposal operation this technical solution is proved to be quite expensive and hence not viable economically, and

(b) Introduction of a ‘microbiological solution’ — may prove to be an ultimate satisfactory answer in the delivery of a clear effluent from the final settling basins.

Special Point : In the past several decades it has been observed that there has been a drastic and remarkable decrease in the content of ‘organic material’ present in industrial effluents. Indeed this spectacular and wonderful accomplishment was duly possible due to the concerted efforts of extensive application of research and development amalgamated with an appreciable volume of expenditures with regard to newer treatment installations.