The sewage can be applied to the land by the following methods: 1. Surface Irrigation 2. Sub-Soil Irrigation 3. Flooding 4. Ridge and Furrow 5. Spray Irrigation 6. Lagooning.
Method # 1. Surface Irrigation:
In this method the parallel drains are constructed in the fields. All these drains are connected to a distributary drain by means of regulating device so that sewage can flow in the required drain. This method is most suitable in sloppy areas.
The sewage is allowed to overflow through fields from one drain towards another as shown in Fig. 11.2. When sewage flows over the fields, its large quantity is absorbed by the fields and only the excess quantity reaches another drain. From the second drain the sewage is allowed to overflow through another area.
Method # 2. Sub-Soil Irrigation:
In this method a network of porous open joint pipes is laid about 30 cm. below the ground level. The sewage is allowed to flow through these pipes which is absorbed by the sub-soil. The remaining quantity of sewage, if any, can be used for irrigation to another place or discharged in natural water courses (Fig. 11.3).
Method # 3. Flooding:
The irrigation area is divided into various parts surrounded by dykes. The sewage is filled like small ponds in between the dykes as shown in Fig. 11.3. The depth of sewage. Flooded over the fields varies from few centimetres to 50 cm. depending on the requirements of the irrigation.
Method # 4. Ridge and Furrow:
In this method the land is first ploughed deep up to 30 cm, levelled and divided into plots and subplots. Then each sub-plot is enclosed by small dykes. Now ridges and follows are formed in each subplot. The sewage is allowed to flow in furrows, whereas crops are grown on rigids as shown in Fig. 11.5. After an interval of 8-10 days the sewage can be again applied depending on the crops requirement and the nature of soil.
Method # 5. Spray Irrigation:
This method is not used in India. In this method, first the sewage is filled in tanks so that settle able soils may settle. Then the sewage is sprayed over the fields by pumping it through pipes fitted with nozzles at the other end.
Method # 6. Lagooning:
In this method the sewage sludge is allowed to go in a water-tight pond, where a detention period of one to two months is given. Within this period, the sewage sludges are stabilized and dried. During stabilization anerobic action takes place due to which foul gases are produced, therefore lagoons should be constructed much away from the towns and cities from where the smelly wind should not pollute the general atmosphere. Lagooning are less common now-a-days.