# Sedimentation of Raw Sewage: Investigations For a Pumping Station in Northern Germany under Energy-Efficient Pump Control

^{*}

## Abstract

**:**

_{s}) distribution, the impact of energy-efficient flow control on sediment formation in pressure pipes (600 mm diameter) was quantified in comparison to a simple on/off operation. In parallel, the sediment formation for 2 years of pumping operation was monitored indirectly via the friction losses. For the investigated case, settling is strongly influenced by the inflow condition (dry, combined from road runoff). Under combined inflow, the proportion of solids with v

_{s}from 0.007 to 1.43 mm/s significantly increases. In energy-efficient mode with smoother operation and shorter switch-off sequences, the sediment formation is significant lower. The mean deposit’s height in energy-efficient control was calculated to 0.137 m, while in on/off operation the mean deposit’s height was 0.174 m. No disadvantages arise over a long period by installing the energy-efficient control. The decreased flow lead under the investigated conditions even to a reduced sediment formation.

## 1. Introduction

_{s}) enable a continuous description of accumulated mass fraction as function of the respective settling velocity.

- Investigation of the settling behavior of typical raw sewage at the inflow side of an urban influenced PS;
- Evaluation of the modification of the settling behavior by wet weather inflow (road runoff);
- Calculation of the sediment formation inside the connected pressure pipe in (i) two-point control and (ii) energy-efficient control; and
- Verification by determining the deposit’s height inside the pressure pipe by a daily observation of the pipe curve.

## 2. Materials and Methods

#### 2.1. Pumping Station, Control Modes, and Monitoring

#### 2.1.1. Pumping Station

#### 2.1.2. Two-Point Control

#### 2.1.3. Rule-Based Control

#### 2.1.4. Monitoring

^{−1}), torque (Nm), voltage (V). On the outflow side of the PS: pressure (bar), flow (l/s), TSS (mg/L). Power input, frequency, engine speed, pressure and flow were used in this work for the investigation of sediment formation inside the pressure pipe.

#### 2.2. Sampling and Experimental Procedure

#### 2.2.1. Sampling

_{ref}) and related precipitation data.

#### 2.2.2. Experimental Design and Procedure

_{s}(mm/s), by dividing the sewage sample into n sub-samples with n settling times. The choice of the best suitable settling times varies with sewage characteristics. After several pre-tests, a time range between 15 s and 24 h settling was found to cover the expectable range of settling velocities (Table 3), tending to smaller intervals at the beginning, according to the high settling dynamics of wet weather samples.

- Filling tank and columns: Storage tank is filled with the sewage sample from the PS (30 L). 7 settling columns (each volume V
_{c}= 2.415 L, height H_{c}= 380 mm) are filled subsequently via separate tubes by gravity from tank till overflow. Settling starts immediately. - Settling: Parallel settling in all 7 columns until the specific settling duration t reached.
- Sampling and TSS analysis: After the specific settling duration t, a sample (volume = 0.345 L) is extracted from the bottom of the respective column i and analysed for TSS. The samples were extracted using a central vacuum pump, connected to a 10 mm glass tube, ending 10 mm above columns bottom.

_{ref}multiplied by V

_{c}) must correspond to the sum of the ∆TSS

_{i}(TSS

_{i}minus TSS

_{i − 1}) of each column i multiplied by sampling volume (100% settling assumed), see Equation (1). This relationship is helpful to define the required maximal settling time and for error evaluation.

_{s}), Equation (4), describing the cumulative settling velocity distribution.

#### 2.3. Calculation of Sediment Formation and Deposit’s Height in the Pressure Pipe

#### 2.3.1. Sediment Formation

_{s}(mass settled), is defined as the cumulative settled mass within each switch-off sequence. The settled mass is based on: (i) the settling behavior of the raw sewage expressed by experimentally derived F(v

_{s}) function and (ii) the cumulative settling duration t

_{s}during each switch-off sequence. For a comparison of the control modes, the mean cumulative settling duration $\overline{{t}_{s}}$ (s) is calculated for all switched-off sequences over the studied period (Equation (5), where ${n}_{{t}_{s}}$ is the number of logged data points in each switch-off sequence). A diurnal course of the settling duration inside the pipe is the result.

_{s}(mm/s) larger than required to pass the pipe diameter within the mean cumulative settling duration $\overline{{t}_{s}}$ (termed v

_{s,pipe}, settling velocity threshold of the pipe), are defined to form sediments. The related mean settled mass $\overline{{m}_{S}}$ (%) is calculated using Equation (6).

_{s,pipe}, the settling velocity threshold of the pipe calculated by Equation (7).

#### 2.3.2. Deposit’s Height

_{T}(m²)) and (ii) the cross section occupied by sediments (A

_{s}(m²)). Applying the geometric relationships of a circle, the area (A

_{T}), wetted perimeter (P

_{T}(m)) (see Figure 2) and hydraulic diameter (D

_{T}(m)) of free cross section are calculated with Equations (8) to (10).

_{A}(Q)) describing the current pipe curve by Equations (11) to (12), with fitting parameters a

_{0}, a

_{1}and a

_{2.}.

_{s}(m) is estimated solving Equation (13).

_{geo}(m) and h

_{r,i}(m), the calculated friction loss according to Darcy–Weisbach, by Equation (14).

_{T}(m), g the acceleration due to gravity (m/s²), and v the flow velocity (m/s). v can be replaced by the flow rate Q (m³/h) and the free cross section for transport A

_{T}(m²).

## 3. Results and Discussion

#### 3.1. Monitoring the Control Modes

#### 3.2. Settling Properties of Dry and Wet Weather Samples

_{s}). The resulting F(v

_{s}) for dry and wet weather sample (from Table 2), are shown in Figure 4 (line plot). The boxplots indicate the determined settling characteristics of the sewage inflow of PS Rostock–Schmarl due to numerous experiments.

_{s}) curves, is characterized by fitting results with a minimum R² of 0.98 and a maximum root-mean-square error of 0.037. This enables well founded statements about the settling characteristics of the raw sewage inflow. An in depth investigation of the experimental procedure, provided by [23], has already shown the applicability to dry- and wet-weather sewage samples as well as the reproducibility of the experimental procedure.

_{s}-class, from dry to wet curves. Especially the proportion of particles with v

_{s}from 0.007 mm/s up to 1.43 mm/s increases (in sum 54.5%). It shows, that primarily medium-speed particles were washed off and entered the sewer network. The proportion of particles with v

_{s}≥ 1.43 mm/s (fast settling particles) increases only marginally (in sum 10.94%). The proportion of the slowest particle class decreases drastically (−65.43%), according to the increase of the medium speed fraction. The key message is here: the particle spectrum did not change from dry- to wet-weather inflow (same v

_{s}-classes observed), but rather the proportion of particles, especially in the medium speed fraction.

_{s}) curves to the literature ([8,23,24]), the particulate matter of the collected samples at PS Rostock–Schmarl settles significant slower, regardless to the inflow condition (dry, wet). This might be to design of the PS upstream sewerage system, as a separating sewer but with a main road runoff connected. Typical sewerage systems investigated in the literature are namely combined sewers or storm water systems ([8,9,23,24]).

_{s}≥ $\overline{{v}_{s,pipe}}$, settle completely. With all four $\overline{{v}_{s,pipe}}$ values lying in the range of the medium speed particle classes, the changed particle composition due to the combined inflow affects the settling processes in both control modes, but significantly reduced in rule-based mode. In rule-based control, higher $\overline{{v}_{s,pipe}}$ is the result of the formulated control rules (with pump flow = inflow, 17.13 h parallel mode in the studied period). Especially for a combined inflow, with pipe flow = inflow, the formulated control rules leading into shorter parallel pumping sequences (in sum 17.13 h in one year). In two-point control, pumps switch earlier into parallel mode (with pump flow >> inflow, in sum 141.7 h in one year), resulting in a lower $\overline{{v}_{s,pipe}}$ and a high sediment formation potential. Accordingly, a significant reduction in deposit’s formation has to be expected for combined inflow, while a slightly change for dry weather inflow is assumed.

#### 3.3. Sediment Formation

_{s,pipe}(second row). Particle classes with a v

_{s}> current v

_{s,pipe}settle completely. Knowing the settling properties of the transported raw sewage (F(v

_{s})) leads to the sediment formation profile, using Equation (6), see Figure 5, third row.

_{s}threshold profiles are changing accordingly. Consistently, higher values resulting, with v

_{s,pipe}≥ 0.5 mm/s in the night phases (while < 0.1 mm/s in two-point control). From ≈ 09:00 to 23:00 v

_{s,pipe}does not fall below 0.74 mm/s, with peaks up to 2.25 mm/s in the afternoon. Under combined inflow, nearly the same v

_{s,pipe}profile occurs, with peaks of 3.5 mm/s in the afternoon. High v

_{s,pipe}values in two-point mode were only reached during phases of high inflow, especially under combined inflow with peaks up to 1.75 mm/s. However, higher v

_{s,pipe}values under combined inflow (for both control modes) are counteracted by the faster settling characteristics during rain events. The respective sediment formation profiles clearly shows this situation. The sediment formation is dominated by the settling characteristics of the sewage.

_{s}= 63% (two-point control) to m

_{s}= 39.4% (rule-based control). Assuming the TSS content from

_{pipe}× l

_{pipe}× TSS

_{wet}× 0.63) to ≈ 240 kg (… × 0.394) by 158 kg. In contrast, the peak deposit’s decrease for dry weather inflow from ≈ 93 kg (… × TSS

_{dry}× 0.2) to ≈ 70 kg (… × 0.156) by only 23 kg. The difference in the control modes becomes clearer with the mean mass on pipes bottom: under rule-based control the mean mass calculates to 7.5% (dry weather inflow) and 24.2% (wet weather inflow), while under two-point control 15.4% of the initial TSS content reaching the pipes bottom under dry weather inflow and 49.3% under combined inflow. On average, the sediment formation is halved by the rule-based control.

_{dry}× 0.036) mode and 27.6 kg in two-point control (…× TSS

_{dry}× 0.06).

#### 3.4. Verification by Calculated Deposit’s Height

_{s}, by Equation (13), shows the sediment layer thickness over the studied period of 2 years, see Figure 6.

_{s}= 0.1375 m, combined h

_{s}= 0.1372 m, two-point dry h

_{s}= 0.1762 m, combined h

_{s}= 0.1721 m. In rule-based mode, h

_{s}is significantly reduced, due to longer pump intervals.

_{s}. Sediments washed-off from main roads or eroded inside an upstream sewer spilled to the PS, changing the settling characteristics and increasing the deposit’s height long lasting. Apparently, the sediments height increases especially the day after a rain event. Figure 7 shows the change of the deposit’s height by specific rain events for both control modes. The deposit’s height increases with rain events (at day 0), but especially after intense ones with reaching its peak one day after the rain event (day one). Within the two-point control, the deposit’s change at day 0 is at +5 cm, while one day after this change is at +5.5 to 6 cm (0.5 cm to 1 cm further increase to day 0). On the following days, h

_{s}decreases slowly (−3 cm to day one). However, h

_{s}is still higher compared to the day before the rain event (+3 cm). h

_{s}at day 0 is reduced to +2 cm, while one day after a rain event h

_{s}increases again by 1 cm (+3 cm in total). On day 2 h

_{s}decreases by 2 cm, but is still 1 cm higher to the initial value. So, the h

_{s}increase is halved by the control rules (if inflow > optimal flow, then operate at inflow) and the main effect itself is limited to day one after rain events.

_{s}in both control modes up to several days, there are two fundamental findings: (i) washed-off particles entering the upstream sewer were not entirely spilled to the PS within the storm flow. Deposited particles in the upstream links are then spilled progressively to the PS, which creates the further increase at day one after the rain event. This effect is significantly reduced in rule-based mode. (ii) Slowly decrease of h

_{s}starts after peak h

_{s}increase reached at day one after a rain event, independently to the intensity of the event. Due to a stronger h

_{s}increase in two-point control, this decrease phase lasts longer compared to rule-based mode. This long lasting change of h

_{s}leads to a higher power consumption, due to a reduced cross section.

## 4. Conclusions

- The adapted experimental setup (VICAS/VICPOL-protocol) is robust and provides reproducible results;
- Settling behavior from dry to combined inflow changes significantly in medium speed classes;
- Pump pauses decrease/pump sequences increase from conventional two-point control to rule-based control;
- Rule-based mode allows higher threshold values of settling velocity, especially for combined inflow, followed by a significant reduction of the sediment formation;
- Pipe curve observation leads to reliable findings about the deposit’s height;
- Daily variations in deposit’s height were registered, but no significant increase in sedimentation risk.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Designed and constructed experimental setup for settling measurement of raw sewage at University of Rostock.

**Figure 2.**Definition of a pressure pipes geometric parameters for the calculation of the deposit’s height, A

_{T}= free cross section for transport (m²), A

_{s}= deposit’s cross section (m²), P

_{T}= wetted perimeter (m), P

_{s}= deposit’s perimeter (m), w = deposit’s width (m), h

_{s}= deposit’s height (m), d = pipe diameter (m).

**Figure 3.**Results after 2 years monitoring PS Rostock–Schmarl: daily settling and pump duration as stacked bar graph, frequency hydrograph and precipitation including marks for sampling dates.

**Figure 4.**Results for settling measurement of sewage samples from PS Rostock–Schmarl, exemplary settling velocity curves F(v

_{s}) for a wet (

**a**) and dry (

**b**) weather sample including fit results, boxplots with results for all collected samples and average settling velocity thresholds of two-point- and rule-based pump control.

**Figure 5.**Results for sediment formation calculation after 2 years monitoring, average diurnal courses of the settling duration (

**a**,

**b**), settling velocity threshold (

**c**,

**d**) and settled mass (m

_{s}) on pipes bottom (

**e**,

**f**), for rule-based- (

**a**,

**c**,

**e**) and two-point control (

**b**,

**d**,

**f**).

**Figure 6.**Results for calculation of the deposit’s height after 2 years monitoring, settling factor s

_{f}(-) as ratio between the daily settling and pump duration and precipitation P (mm/d) (

**a**) and deposit’s height h

_{s}(m) including simple moving average over 14 d (

**b**). a) significant increase of h

_{s}after intense rain (15.2 mm/d), b) h

_{s}reduced by parallel pumping (high s

_{f}), c) increased h

_{s}after intense rain (16.2 mm/d) in rule based mode, d) increased h

_{s}after re-installing two-point control in parallel with intense rain (33mm/5d), e) decrease of h

_{s}by parallel pumping after long rule-based period with smooth h

_{s}increase, f) fast increase of h

_{s}after re-installing two-point control, g) h

_{s}normalized under two-point control mode to initial values.

**Figure 7.**Results for the calculation of the delayed increase of the deposit’s height h

_{s}inside the pressure pipe after specific rain events for rule-based (

**a**) and two-point control (

**b**).

Control Strategy | On-Level | Off-Level | Operation at Frequency | Soft Start Duration | Duty Point | Control Rules | |
---|---|---|---|---|---|---|---|

Q | H | ||||||

(m) | (m) | (Hz) | (s) | (l/s) | (m) | ||

Two-Point Control | 0.8 | 0.4 | 43–45 | 60 | 130 | 17.8 | transport-rules |

Rule-Based Control | 0.8 | 0.4 | 41 (energy optimum) | 60 | 110 | 17.45 | energy- & transport-rules |

Sample | Sampling Time | Sampling Volume (L) | TSS_{ref} (mg/L) | Remarks |
---|---|---|---|---|

dry | 10:00 | 30 | 390.2 | Dry weather inflow |

wet | 13:00 | 30 | 544.8 | Wet weather inflow: Precipitation height = 10.1 mm/d |

Column | i | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
---|---|---|---|---|---|---|---|---|

Settling Duration t | (s) | 15 | 45 | 180 | 420 | 1800 | 14,400 | 86,400 |

Settling Velocity v_{s} (H_{c}/t) | (mm/s) | 25.3 | 8.4 | 2.1 | 0.9 | 0.2 | 0.026 | 0.004 |

**Table 4.**Results for settling measurement of sewage samples from PS Rostock–Schmarl, average change in class proportion from dry to wet samples.

v_{s} range | F(v_{s})_{wet} − F(v_{s})_{dry} |
---|---|

(mm/s) | (%) |

v_{s} ≥ 40 | +1.19 |

13.3 ≤ v_{s} ≤ 40 | +1.55 |

3.3 ≤ v_{s} < 13.3 | +3.72 |

1.43 ≤ v_{s} < 3.3 | +4.49 |

0.33 ≤ v_{s} < 1.43 | +17.19 |

0.04 ≤ v_{s} < 0.33 | +22.98 |

0.007 ≤ v_{s} < 0.04 | +14.33 |

v_{s} < 0.007 | −65.43 |

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**MDPI and ACS Style**

Rinas, M.; Tränckner, J.; Koegst, T.
Sedimentation of Raw Sewage: Investigations For a Pumping Station in Northern Germany under Energy-Efficient Pump Control. *Water* **2019**, *11*, 40.
https://doi.org/10.3390/w11010040

**AMA Style**

Rinas M, Tränckner J, Koegst T.
Sedimentation of Raw Sewage: Investigations For a Pumping Station in Northern Germany under Energy-Efficient Pump Control. *Water*. 2019; 11(1):40.
https://doi.org/10.3390/w11010040

**Chicago/Turabian Style**

Rinas, Martin, Jens Tränckner, and Thilo Koegst.
2019. "Sedimentation of Raw Sewage: Investigations For a Pumping Station in Northern Germany under Energy-Efficient Pump Control" *Water* 11, no. 1: 40.
https://doi.org/10.3390/w11010040