Organic Waste Recycling

458 Organic Waste Recycling: Technology and Management

In addition to Table 9.12, the U.S. EPA also classifies sludge quality based on
indicator bacteria. The class A indicator standard is less than 1000 faecal coliforms
per gram of dry solids and the class B indicator standard is less than 3 million
faecal coliforms per gram of dry sludge solids (Walsh 1995). If a sludge contains
about 1% solids, the class A standard is apparently similar to the WHO standard for
unrestricted irrigation (Table 2.27), and there are no use restrictions placed on
sludge that meet the class A requirements. Because the class B sludge poses greater
health risks, the U.S.EPA imposes more use restrictions than those outlined in
Table 9.12, e.g.

1. food crops that receive a sludge application cannot be harvested for
   periods ranging from 14 – 38 months afterwards, depending on the
   type of crop grown and the method of application


2. pasture lands that receive sludge cannot be grazed for 30 days


3. turflands are not allowed to be harvested within 12 months of
   application, and


4. public lands that receive sludge application will have access
   restricted for 30 days in low-exposure area and up to 1 year for high
   exposure areas.

Table 9.13 Concentration limits of heavy metals in sewage sludge used for land
application in some European countries (cited in Rulkens et al. 1989). Reproduced by
permission of Chapman and Hall

Concentration Limit (mg/kg dry solids)
Zn Cu Pb Cr Ni Cd Hg As
Netherlands
France
Germany
Norway
Sweden
Finland
Scotland

2,000

3,000

3,000

3,000

10,000

5,000

10,000

600

1,000

1,200

1,500

3,000

3,000

1,000

500

800

1,200

300

300

1,200

800

500

1,000

1,200

200

1,000

1,000

800

100

200

200

100

500

500

250

5

40

20

10

15

30

20

5

10

25

7

8

25

7.5

10

• 
  


• 
  


• 
  


• 
  


• 
  

  150

  
  

9.3.1 Sludge Application Rates

The application rates of sludge to cropland vary according to the nutrient
requirements of the crop, existing soil characteristics (e.g. drainage, nutrient
level, heavy metal content), climate, and the characteristics of the sludge. In all
cases, the application rate should be such that:

1. Crop production and quality are not decreased.


2. The soil does not build up excessive organic material or heavy
   metals.