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