## 4 Conclusions  
In this paper, the feasibility of novel acoustic instrumentation to detect changes in the combined sewer overflow (CSO) screen condition was studied. The results obtained from two field trials (phases 1 and 2) deployed in urban residential areas provide an informed decision when the screen needs to be cleaned. The controlled field experiments in phase 1 suggest that the change between the clean and fully blinded CSO screen conditions is detectable with the proposed acoustic method. In the case of the smaller screen (site 1), this change was reflected in the 8% increase in the absolute value of the coefficient \(b_{0}\) in Equation (3). In the case of the larger screen, the corresponding increase in the absolute value of this coefficient was 39%. These changes are clearly detectable with the proposed acoustic method. This acoustic instrument, installed to monitor a plurality of types of CSO, will require initial calibration--a straightforward process which can be automated and completed remotely. In phase 2, the data from the acoustic experiment in the field were compared against the Hawkeye water level data obtained in the same CSO chamber simultaneously, albeit at a different sampling interval. The acoustic sensor head worked properly over a period of six months. The reproducibility of the acoustic sensor data during the dry weather flow period was found within \(\pm 2.5\%\). This level of repeatability, together with obtained experimental evidence, suggest that the system can reliably detect changes in the conditions in the CSO screen which were caused by a rapid change in the water level. A deposition of debris on the CSO screen by a short-term storm event caused a 30% increase in the absolute value of coefficient \(b_{0}\) in Equation (3) and a 5 dB reduction in coefficient \(a_{0}\). These coefficients have recovered to their ambient values after the screen returned naturally to its original condition. The results also show that the sensor can detect water level changes which occur over a period of time, which is longer than the sensor sampling interval. A significant increase in the water level causes a measurable drop in the sound pressure level in the chamber and reduction in the acoustic attenuation. These results make physical sense. It was found that the acoustic sensor performance returns to normal after the level of water drops to its ambient dry weather conditions. The speed of the acoustic measurements (a few seconds) and sensor's ability to quantify the screen condition remotely have the potential to be at least two orders of magnitude less costly than traditional site visit and visual inspection methods. The results from phase 2 experiments show that the screen condition can naturally recover after a short-term storm event, which usually causes some debris deposition. This information is novel as it has not been observed before. This implies that the deployment of such sensors can help to reduce unnecessary site inspection visits, resulting in a reduction of operational costs and associated environmental impact. The proposed instrument can be improved further by pairing it with an energy harvesting system (e.g., solar or wind) and integrating it with the wireless network, e.g., the Hawkeye system. Conceptualization, C.H.S., K.V.H., M.T.B.A. and S.J.T. ; methodology, C.H.S., M.T.B.A. and K.V.H. ; software, M.T.B.A. ; validation, C.H.S., K.V.H. and M.T.B.A. ; formal analysis, C.H.S. and K.V.H. ; investigation, C.H.S. and K.V.H. ; resources, K.V.H. ; data curation, C.H.S. and M.T.B.A. ; writing--original draft preparation, C.H.S. ; writing--review and editing, C.H.S., K.V.H., S.J.T., M.T.B.A. ; supervision, S.J.T. and K.V.H. ; project administration, S.J.T. and K.V.H. ; funding acquisition, S.J.T. and K.V.H. All authors have read and agreed to the published version of the manuscript.