Organic Waste Recycling

Land treatment of sludge 447

The physical characteristics of various types of sludge are given in Table 9.2.
Well-digested sludge is inoffensive and quite suitable for land application. Other
types of sludge are mostly not well stabilized and have offensive odours.
Typical chemical compositions of raw and digested sludge are presented in
Table 9.3. Digested sludge usually contains less percentage of volatile solids
and protein than raw sludge, however the former types of sludge have high
contents of nutrients (N, P, and K). Table 9.4 gives the nutrient contents of
several sludge types in USA, which indicates the suitability of using this sludge
in agricultural production and land reclamation. Since many municipalities can
offer their sludge at practically no cost to the farmers, a significant cost saving is
effected both for the farmers and the municipality which produces the sludge.
However, the cost of sludge transportation from a treatment plant to a land
application site and proper application of sludge to land (see section 9.2) can be
expensive, in some cases.

Table 9.2 Physical characteristics of various types of sludge (Loehr and Jewell 1979)

Sludge Colour Other
physical
properties

Odour Digestibility
(amenability to
further biological
stabilisation)
Primary
sedimentation
Chemical
precipitation
(primary)

Activated
sludge

Trickling filter
humus

Digested
sludge

Septic tank
sludge

Grey

Black, red

surface if

high in iron

Brown, dark

if nearly

septic

Brownish

Dark brown

to black

Black

Slimy

Slimy,

gelatinous,

gives off

considerable

gas

Flocculent

Flocculent

Contains

very large

quantity of

gas

Extremely

offensive

Offensive

Inoffensive,

earthy when

fresh, putrefies

rapidly

Relatively

inoffensive,

decomposes

slowly

Inoffensive if

thoroughly

digested; like

tar or loamy

soil

Offensive

(H 2 S) unless

very long

storage time

Readily digested

Slower rate than

primary

sedimentation

Readily digested

Readily digested

Well stabilized

Mostly stabilized