Smart sludge management solution for small-scale plants – humification fields in Türi and Oisu WWTPs
A pilot investment was done in Türi (8000 PE) and Oisu (500 PE) wastewater treatment plants (Estonia) in order to show a good example of efficient sludge treatment for small-scale plants by applying the technology of sludge humification beds. This technology allows treating sludge in a natural way without using any complicated mechanical facilities or chemicals, simultaneously ensuring low constructional and operational costs and low environmental impact. Sludge humification is one of the solutions that provide an option for the phosphorus and nitrogen reuse: it is applicable on the small scale, it helps with decreasing and eliminating harmful substances and organisms from the sludge, and it allows using the treated sludge as a fertilizer.
Different systems for humification were applied for Türi and Oisu WWTPs – the output of Türi WWTP is dewatered sludge with higher solids content (around 16-17% TS) and therefore can be treated differently than the output of Oisu WWTP (around 1% TS). In Oisu, the beds were equipped with a leachate pumping system to lessen the amount of water in the humification beds during dewatering and plant growth period. In Türi, pumping away the excess water is not required, as mechanical thickening and dewatering are applied beforehand. The humification beds in both WWTPs are sowed with plants, which helps with the degradation of organic material, including refractory compounds; while also provides the ability to absorb the residues of toxic substances like heavy metals from the sludge. In order to extract the toxic substances absorbed in the plants, mowing or reaping can be used.
Components installed in the solution
In Türi WWTP: existing asphalted area was expanded by 900 m2 (to a total of 3000 m2) and runoff collection was designed, where dewatered sludge is laid out and seeded with ryegrass for the humification process.
In Oisu WWTP: 2 humification beds with the volume of around 40 m3 each, together with a leachate pumping station to increase the solids content of sludge. The gravity thickened sludge is pumped directly to the beds, where ryegrass is seeded for the humification process.
Considering that sludge management in humification beds is a long-term process lasting at least three vegetation periods, the time- and site-based planning based on preceding studies is an essential part of the piloting process. The humification beds in Türi and Oisu were constructed next to the wastewater treatment plants and they have the same working principle, while their structure is a bit different in order to determine the most feasible practice.
Small-scale piloting of the humification process was also carried out at Türi WWTP to make sure the climate and other conditions are feasible for the sludge treatment. The preliminary results have shown both an increase in total solids content (from 16% TS to 32% TS in 3 years) and a significant increase in the mineral content in solids (from 20% of mineral content of TS to 50% in 3 years). The data collection of the full-scale investments in Türi and Oisu WWTPs will begin during Spring 2019.