Descriptive Assessment and Amendment of the SimpleTreat Model - Modelling of Organic Chemicals in Sludge for Soil Applications

Abstract: Large quantities and numbers of organic compounds (OCs) from industries and domestic sources enter the wastewater treatment plants (WWTPs) everyday, which is an ever-increasing issue in modern society. In WWTPs, the pollutants are either degraded or emitted to the air, the effluent (water) or the sludge (biosolids). Even though the sludge may contain toxic contaminants and heavy metals, there is an incentive of recycling sewage sludge as a fertiliser to preserve the phosphorus that is a limiting resource. To ensure the quality of the sludge, the WWTPs can certify their work according to the Swedish certification system, REVAQ that includes setting up an upstream action plan (uppströmsarbete in Swedish) by which to improve the sludge. Nevertheless, the overall environmental risk of OCs, i.e. from entering the WWTP to exposing the sludge containing OCs to soil processes, has so far not been taken into consideration in the upstream action. This master’s thesis is carried out at SWECO Environment AB in association with the umbrella project “Organic compounds in sludge recycling, evaluation and prioritisation” for the Swedish Water & Wastewater Association, in cooperation with four Swedish wastewater treatment plants: Ellinge, Käppala, Ryaverket and Sjölunda. The main objective of the thesis is to develop a methodology to predict the fate of organic chemical pollutants in sludge after being exposed to soil processes by applying and amending the easy accessible modelling tool SimpleTreat 3.1. However, further investigation of how the soil processes will affect the removal of the OCs in soil will not be assessed in this thesis. The SimpleTreat will as well be adapted to existing known parameters of generic Swedish WWTPs that have not been included in the original SimpleTreat. In addition, the applicability of the modified model for four generic Swedish WWTP configurations will be validated by comparing the predicted results of effluent and sludge with measurements from the IVL-database (Swedish Environmental Research Institute). Furthermore, by simulating measured chemicals in the influent, a chemical list with expected concentration ranges in Swedish effluent and sludge can be obtained, especially when many of the emerging chemicals’ presence, frequency of occurrence or source may be unknown. Since two input parameters from Ellinge are missing, this WWTP has consequently been left out of the investigation. The old and new sections of Käppala have different plant configurations and are therefore treated as two separate plants. In summary, a stepwise methodology presented to predict the distribution of OCs to water and sludge, which comprises of a plant description, chemical data collection, calibration and validation. The OCs that are investigated and measured in WWTPs are mostly hydrophobic and their emissions to air are thus negligible. Moreover, the SimpleTreat model was modified and proved to be applicable for predicting the fate of organic pollutants with regard to a number of factors: 1) the model structure, 2) the model parameters and 3) the data quality. Firstly, the structure of the modified SimpleTreat is limited to a primary settler, an activated sludge system (anaerobic, anoxic and aerobic zones) and a secondary clarifier. Secondly, the physico-chemical parameters are preferably experimentally measured but estimated values from EPI (Estimation Programs Interface) Suite™ are equally satisfactory. Thirdly, the data of influent, effluent and sludge has to be measured at the right location within a reasonable time frame and employing the same sampling and analysis method. To improve the results for a specific compound, it is suggested to measure the partition coefficient Kp in raw sewage, primary settler, aeration tank and secondary clarifier to gain a more precise prediction of the partitioning in the defined WWTP. In conclusion, a total of 84 chemicals were modelled in the modified SimpleTreat with the four plant configurations respectively; the chosen WWTPs attested representative overall pollution removal rates via effluent and sludge when compared with typical concentrations ranges of the IVL-database. Predictions of the concentrations’ order of magnitude in Swedish effluent using the modified SimpleTreat proved to have 92% accuracy and in Swedish sludge 56% accuracy. However, further analysis of the chemicals must be carried out to classify the risk of the chemical in soil amendment. Finally, the information that can be acquired by applying all steps in the methodology is which substances that do not pose a threat to the environment and human health despite being recycled with sludge, supposing that the concentration level of toxicity is known.