From the above comparative analysis, it is evident that there are few
parameters where the STP compost is not as good as in-house compost and
vice versa. The moisture content is way higher for the STP compost than
the prescribed limits. The major reason for this is that the sludge was
collected on a rainy day and as it is lying open in the plant premises
it is quite usual for it to get wet. However, this can be easily managed
with regular drying technologies. The organic carbon content is above
15% for both in-house and STP sludge compost against the specification
of 14%. Interestingly, the pH of the sludge compost is within the
prescribed specifications as compared to the pH of in-house compost.
This is beneficial as it would not require external additions to bring
the pH within the limit thereby saving cost and preserving the chemical
nature of the compost. The C/N ratio for the sludge compost is much
higher than the specifications which is due to the low N content. It can
be seen that the N content is just marginally higher than the prescribed
specification in the case of the sewage sludge but the in-house compost
is of better quality in this respect. The phosphate content is almost
double the specifications in the case of sludge compost. The phosphate
content is approximately 0.8% for in-house compost as compared to a
specification of 0.5%. The Potassium content of STP sludge compost is
way lower than the recommended specification. The in-house compost also
does not contain the desired concentration. The total NPK content is
also lower than the specifications for both in-house and sludge compost.
Overall, we find that the STP sludge contains a number of valuable
nutrients. Most of them are in concentrations greater than the desired
specifications. The presence of plant macronutrients also makes it an
important constituent that may be used in addition to other inputs so as
to utilize those for plant growth. Fortification of the sludge compost
with correct additives will enhance its properties and make it a
fruitful addition for agriculture.
We have tried to analyze the metal concentrations present in the sludge
as well as in-house compost using Atomic Absorption Spectroscopy. The
macro and micronutrient status were also measured and analyzed. The
table 2 below shows the results of heavy metals in STP and in-house
sludge in comparison to the desired concentration as specified by
government regulations (Government of India, 1985).
Table 2: Concentration of metals in the sludge