URBAN SITE Gaps detection

Out of the 1 052 640 minutes (731 days) covered by the records (chapter 4), 66 719 minutes were not saved (6

% of the duration). They are divided in 339 gaps which duration ranges from 1 to 60 474 minutes. Most of the gaps are short: 85 % of the gaps are less than or equal to 10 minutes. Only 3.5 % of the gaps are more than 1 hour long. Figure 43 shows the time distribution of the gaps for the recorded 2 years. A simple linear function was used to fill the gaps. The precise cause of missing data is not known, but it is surely linked to malfunctions monitoring system, either the measurement apparatus or the data transmission or banking.

Figure 43: Bellecombe WWTP daily inflows and rainfall in 2012 and 2013.

Daily flows

The daily flows are very variable (from 2 352 to 14 400 m³ per day). A comparison with the recorded rain events on the catchment tends to indicate that the network is actually draining a significant amount of parasitic water (figure 43). Indeed, the Pearson product-moment correlation coefficient between the increase of flow from one day to another (excluding the days with smaller daily flow than the previous day) and the rainfall of the same day, the day before and 2 days earlier are respectively 0.30, 0.72 and 0.09. This means that the increases of the daily wastewater flow are correlated to the rainfall of the same day and the day before. In addition, the daily flows are slowly decreasing after wet weather days. As it is highly improbable for pharmaceutical products to come from rain and runoff, it is important to identify and quantify the parasitic water flow for dry weather days.

A simple method is proposed. Assuming that the evolution of the parasitic water flow is slow (evolution over

night time (0 to 7 h), it is possible to build a “parasitic water baseline” by detecting the minimum flow of each night of the time series and linearly linked them. To account for the presence of “non-parasitic” wastewater during night time, 0.15 m³/min are subtracted for each minute from the “parasitic water baseline”. It corresponds to roughly one toilet flush per inhabitant per night and some non-domestic wastewater flow spread over the 7 hours of night time (it represents 3 % of the expected production of wastewater by the Bellecombe catchment). The results for the wastewater time series of Bellecombe are shown in figure 44.

Figure 44: Daily parasitic water at the inlet of the Bellecombe WWTP for the 2 recorded years.

Subtracting the infiltration baseline from the wastewater flow time series, and only considering days with no long gaps in data (longest gap <10 min) and with dry weather from the day before to the day after (rainfall <0.5 mm for the three days), the daily flows are analysed (figure 45). For the 211 remaining days, the average daily flow is 2 088 m³/day with a standard deviation of 179 m³/day. Week days and weekend days shows similar patterns with, on average, 4 % more wastewater during weekend days.

Figure 45: Distribution of the daily wastewater flows of the Bellecombe catchment.

Flow dynamics

Looking at an infra-day scale, the main characteristic of the wastewater flow is the rapid and important fluctuations of the flow (example in figure 46). These are the result of the pumping stations in the sewer network. They make the analysis of the wastewater flow dynamics difficult. To overcome this difficulty and get the average dynamics of the wastewater flow, a 30 minutes mobile mean was applied to the whole time series to smooth the fluctuations caused by pumping stations.

Figure 46: One day of the Bellecombe raw wastewater flows time series.

Considering the smoothed time series, subtracting the infiltration baseline and only keeping days with no long gaps in data (longest gap <10 min) and with dry weather from the day before to the day after (rainfall <0.5 mm for the three days), the infra-day dynamics of the wastewater flow of the Bellecombe catchment is analyzed.

The Nash-Sutcliffe Efficiency coefficient is used even if it is not a model, in order to highlight similar or dissimilar dynamics patterns (table 25).

Wastewater flows of week dry days are similar. Indeed, the average NSE of the wastewater flows of week dry days with, as reference time series, the average wastewater flow of week dry days is equal to 0.86. Likewise, the wastewater flows of weekend dry days are similar (NSE of 0.90). However, different dynamics are observed between week and weekend dry days. Indeed, the average NSE of the wastewater flows of week (respectively weekend) dry days with, as reference time series, the average wastewater flow of weekend (respectively week) dry days is equal to 0.55 (respectively 0.36). Representations of both wastewater flows of week and weekend dry days are shown in figure 47.

Table 25: Nash-Sutcliffe efficiency coefficient (NSE) of the wastewater flows at the inlet of the Bellecombe WWTP.

Time series as “modelled” data Time series as “measured” data Average NSE (standard deviation) Wastewater flows of week days Average wastewater flow of week days 0.86 (0.15)

Average wastewater flow of weekend days 0.55 (0.19) Wastewater flows of weekend days Average wastewater flow of week days 0.36 (0.23) Average wastewater flow of weekend days 0.90 (0.05) Wastewater flows of week and weekend dry days share the same basic shape consisting of low flows during the night followed by a rapid increase in the morning and a local minimum flow at the end of the afternoon followed by an evening peak. The main difference concern the morning flows since the rapid increase of the flow are happening at different times and reach different levels (approximately 2 hours later and 860 m³/day more during weekend days). After that the flows seems to slowly catch up until the evening peak that is reached almost at the same time (1 hour sooner during weekend days) with the same level (≈ 3 300 m³/day).

Figure 47: Dynamics of the wastewater flow for the Bellecombe catchment. For week and weekend dry days, the lowest flow happening during night time is approximately equal to 290 m³/day. For week and weekend dry days, the morning increase respectively starts approximately at 6 h 30 and 8 h 30, ends approximately at 8 h and 12 h and reaches 3 460 m³/day and 4 320 m³/day. Evening peaks of 3 310 m³/day are reached at 21 h for week dry days and 20 h for weekend dry days.

Further uses

Wastewater flows of the Bellecombe catchment are used for two purposes:

 Pharmaceuticals loads calculation: for each sampling campaign the actual daily flow is measured directly at the WWTP.

 Calibration and verification of the hydraulic part of the model. As the model simulates wastewater flow during dry weather periods for week days, not all days of the 2012/2013 time series are suitable for calibration or verification of the model. Only periods of two consecutive days are kept. They must not contain gaps in data longer than 10 min during the sampling period (8 h to 8 h), rainfall must be below 0.5 mm and neither day must be weekend days. This provides 129 sets of periods of 2 consecutive days.

6.2.2 CHAL HOSPITAL