Night Soil, Sewage and Sludge

After reading this article you will learn about night soil, sewage and sludge.

Night Soil:

Night soil is human excreta, which is very rich in plant nutrients. Agricultural chemists have found that fresh human faeces contain about 1.5 per cent N, 1.1 per cent P₂O₅, 0.5 per cent K₂O and 1.0 per cent CAO.

Although human urine, which contains about 0.6 per cent N, 0.1 per cent P₂O₅ 0.5 per cent K₂O and 0.3 per cent CaO, can be directly applied to the field, hygienically fresh human faeces cannot be directly applied to the field. This material may be dried in shallow beds after it is covered with soil for about eight to ten days, when it is known as poudrette which contains about 1.1 per cent N, 0.7 per cent P₂O₅ and 0.9 per cent K₂O.

Poudrette may also be prepared in trenches of about twelve feet length three feet width and one foot deep. Night soil is uniformly spread in a thin layer at the bottom of the pit and is covered with another layer of soil the trench is filled in this manner.

The material dries in the trench in about eight to ten months when it contains about 1.8 per cent N, 1.9 per cent P₂O₅ and 1.1 per cent K₂O. In the Nasik system, organic refuse is added to human excreta. This mixture is then deposited in trenches.

Sewage and Sludge:

Human excreta are flushed by water through underground sewage pipes. This known as the sewerage system. Human faeces and urine mixed with water is known as raw or crude sewage, which consists mainly of a colloidal solution of human excreta rich in the available forms of plant nutrients.

Its application to the field therefore increases crop yields. The colloidal material in raw sewage may choke the soil pores and make the soil sewage sick if it is continuously applied to the field. Therefore, its continuous application to the field should be avoided.

The sewage is treated in either of the following to make it more suitable as manure:

(i) Septic tank method,

(ii) Filter bed method,

(iii) Activated sludge process.

All these processes work on the principle of biological oxidation of faecal material either aerobically or anaerobically; a part of it is converted to methane, hydrogen and ammonia, another part remains in solution and a third part settles down to the bottom which is known as sludge, and the liquid portions known as effluent.

In the septic tanks method, the sewage is left undisturbed in the septic tank, where the faecal material is anaerobically decomposed to evolve considerable amounts of methane, hydrogen and ammonia, although considerable amounts of colloidal faecal materials remain in the effluent.

In the Filter Bed Method, the sewage is allowed to percolate through filters where the faecal material aerobically decomposes. A part of it is converted to gas, another part remains in solution, and a third part is thrown down as the sludge.

The effluent is almost entirely clear. In the Activated Sludge Process, air is forced through the sewage, and almost all the faecal material is oxidized, and only a little settles down to the bottom as sludge, leaving the effluent as clear as water.

The effluent obtained from the filter bed method and the activated sludge process can safely be directly applied to the field, but the septic tank effluent should be carefully used because it still contains lots of objectionable colloidal materials and pathogens.

The effluent is usually rich in ammoniacal nitrogen i n the septic method while the effluent obtained by the activated sludge process is usually rich in nitrates. The sludge is rich in nitrogen and phosphorus and poor in potassium.