Reclamation of Soil

It has already been stated (sec. 4.5.1 and 9.2.7) that acid rain makes the soil acidic. Also, use of excess fertilizers pollutes the soil. For example, when ammonium sulphate is used as fertilizer again and again into the soil, NH₄⁺ ions are used up by successive crops but SO₄^2- ions get accumulated into the soil.

In case NaNO₃ or KNO₃ is used as fertilizer again and again, the N0₃~ ions are used up by the successive crops, but Na⁺ and K⁺ ions get accumulated into the soil. These cations render the soil alkaline and so it becomes unfit for

subsequent crop cultivation.

We also know that fertilizers are used to increase crop production and a lot of water is used for irrigation. In case, there is not good drainage system for the outlet of unused water, the soil becomes highly saline (salt containing). This is because the salts formed get accumulated in the soil. The saline soil becomes unfit for growing crops.

For agricultural crops, the pH of the soil should be near neutral. So acidic or basic soils are not good for cropping. In such a situation, the acidity or alkalinity must be removed from the soil. This is called reclamation of soil.

If the soil becomes too acidic, it may be reclaimed by addition of lime (CaCO₃). This process is called liming.

In drought - prone areas with low rainfall, the soil may turn too alkaline due to the presence ofbasic salts like Na₂CO₃. Alkaline soils can be reclaimed by treating with Al₂(SO₄)₃ or Fe₂(SO₄) ₃ which on hydrolysis sets free the acids.

Alkaline soils can also be reclaimed by addition of sulphur which is oxidized in soil by bacterically mediated reactions to H₂SO₄.

Saline soils can be reclaimed to a desired level by the removal of salts which are responsible for salinity. The procedure consist in leaching the soil with good quantity of water filled by drainage of water. The water dissolves the salts which are thus drained out.

9.6 Treatment of Soil Pollutants

The nature of pollutants depends on the specific industries, e.g., polymers and plastic, asbestos, tobacco, petroleum, mining and agrochemical industries, (for details of pollutants, see secs. 4.3.1; 4.3.2; 4.3.3; 4.3.6). Besides these, the pollutants are also discharged into the land by industries such as textile, pulp and paper, leather, food processing, dairy, soap and detergents etc. (for details, see secs. 6.4.2; 6.4.2.1; 6.4.2.2; 6.4.2.3; 6.4.2.4; 6.4.2.5; 6.4.2.6; 6.4.2.7; 6.4.2.8). The harmful effects of various pollutants and their prevention have also been discussed in the relevant sections mentioned above.

The industrial wastes are the source of soil pollution due to their disposal on the land. The industrial units adopt one or more of the following procedures to take care of their wastes and its disposal.

(i) Treat their wastes to remove suspended materials and then discharge the effluents in the water bodies.

(ii) Discharge untreated effluents into the sewage system which has to be processed with domestic sewage by municipal sewage treatment plants. This, of course, has to be done with the concurrence of the municipalities.

(iii) Dumping of solid wastes into land dumping sites and discharging liquid wastes into sewers.

(iv) The chemical treatment of industrial wastes e.g., coagulation and precipitation to remove suspended matter.

(v) Screening, oxidation, flocculation and sedimentation of effluents followed

j'; by biological treatments.

- ^: (vi) Removal of highly toxic compounds by ion - exchange, electro - dialysis

or absorption by active carbon before their disposal into the land.

ι_ll ^ι The sewage and domestic waste consists of dirty water and contains human

∣ _l j and animal excretions (e.g., urine and faeces). The discharge of large quantities

of sewage into land and water bodies causes pollution. It is, thus, necessary to treat the sewage and domestic waste. The procedure is based on biological :: decomposition of non - toxic organic wastes using bacteria under aerobic

' ³ conditions i.e., in the presence of plenty of oxygen.

.

As a first step, the solid wastes are removed from sewage and domestic waste by screening followed by settling of the sludge. Subsequently, the residual liquid is subjected to biological oxidation of soluble organic material through a I bed of microbes in activated sludge. Any sludge formed is again removed. The

sludge may be used as fertilizer and the remaining water is chlorinated before discharging into water bodies.

9.7 Control of Soil Pollution

(1) Itshouldbemandatoryforindustrialunitsnottodumptheirwastesonto the land. In fact, the industries must ensure that there is minimum of waste products (see sec. on green chemistιy as an alternative for reducing pollution in Chapter 16). As far as possible, the waste products could be recycled or used to make useful products.

(2) The domestic and urban garbage wastes should be properly managed by Ihemunicipalities.

(3) Animal refuge and agricultural wastes can be utilized as manure and for the production ofbiogas - that in turn may help to generate electricity.

(4) Materials like paper, glass, metal scraps and some type of plastics can be recycled.

(5) Biological methods of pest control can also reduce the use of pesticides and so minimize soil pollution.

9.6