Agroecological assessment of wastewater and sludge from Municipal Wastewater Treatment Plant by content of nutrient inputs

The purpose of this study was to investigate and assess the quality of wastewater and sludge from different monitoring points (4 for wastewater and 3 for sludge) in Municipal Wastewater Treatment Plant, Stara Zagora, Bulgaria, for the period July 2013 – March 2014, by content of nutrient inputs (total N, total P and total K), absolutely dry matter, pH and electrical conductivity, in order to determine opportunities for their use in agriculture. For sampling and sample preparation of wastewater and sludge, international references of ISO and BSS were used. Sample analyses were made by equipment as follows: рН-meter Lab 850 (pH), Multi 340i/SET (electrical conductivity), method of Kjeldahl (total N), spectrophotometer UV/VIS 6705 JENWAY (total P) and ААС „AAnalyst 800”– Perkin Elmer (total K). It was found that: 1) Treated wastewater has neutral pH, low electrical conductivity and low total K content, as well as lower content of total N and total P than the maximum allowed concentrations for their discharge into the receiving water body – Bedechka river, defined from ecological point of view as a sensitive water body. 2) The effect of wastewater treatment in respect of total N was 87.4%, total P – 97.8% and total К – 23.0%, respectively. 3) Treated wastewater meets the requirements for irrigation of crops by indices: pH, electrical conductivity and content of total N, total P and total K. 4) The sludge has typical content of absolutely dry matter, neutral pH and high values of electrical conductivity, as well as lower content of total N (from 1.3 to 3.2 times), total P (from 3.2 to 7.8 times) and total K (from 4.4 to 12.2 times) in comparison with the sludge from 9 other WWTPs in the country, which is result of the modern technology of sludge treatment in WWTP-Stara Zagora. 5) Sludge contains more total N (from 2.71 to 7.5 times), comparable amount of total P and less quantity of total K compared with livestock manure.