Vacuum Sewers

Compiled by:

Beat Stauffer (seecon international gmbh)

Source:  http://www.sswm.info/category/implementation-tools/wastewater-collection/hardware/sewers/vacuum-sewers

Executive Summary

Vacuum sewerage systems consist of a vacuum station, where the vacuum is generated, the vacuum pipeline system, collection chambers with collection tanks and interface valve units. In contrast to conventional gravity sewerage systems with intermediate pumping stations, the permanent pressure within the vacuum system is maintained below atmospheric pressure. Moreover, vacuum technology reduces water consumption considerably, enabling flexible installations regardless of topography and water availability. In addition, it allows for the use of alternative wastewater handling (blackwater and greywater separation).

In	Out
Blackwater, Faecal Sludge, Greywater,Brownwater, Urine or Yellowwater, Faeces,Excreta, Energy	Blackwater, Non-biodegradable Wastewater

Basic Design Principles

Overview of a vacuum sewer system. Source: ROEDIGER (2007)

Vacuum sewerage systems use a central vacuum source to convey sewage from individual households to a central collection station (UNEP 2002). It is a mechanised system ofwastewater transport. Unlike gravity flow (e.g.conventional sewers,separate sewers or simplified sewers), vacuum sewers use differential air pressure (negative pressure) to move the sewage. A central source of power to operate vacuum pumps is required to maintain vacuum (negative pressure) on the collection system. The system requires a normally closed vacuum/gravity interface valve at each entry point to seal the lines so that vacuum can be maintained. These valves, located in valve pits, open when a predetermined amount of sewage accumulates in collecting sumps. The resulting differential pressure between atmosphere and vacuum becomes the driving force that propels the sewage towards the vacuum station (PDH ENGINEER 2007).

Transport of Wastewater

  

Steps of wastewater transportation in a vacuum sewer system. Source: PDH ENGINEER (2007) 

A traditional gravity line carries wastewater down to the collection chamber (it is recommended to combine it with vacuum- or low-flush toilets). As soon as the level reaches a defined height, thevacuum interface valve opens and the negative pressure sucks the wastewater into thevacuum sewer main. At the end of the pipe system, there is a collection tank. When the tank fills to a predetermined level, sewage pumps transfer the contents to a treatment plant via aconventional or separate sewer system. It is important to understand that the collection system is hold on permanent level of vacuum.

Collection Chamber

     

Cross-section of a collection chamber. Source: GTZ(2005)     

The wastewater from the houses is held back in collection chambers, with pneumatic regulating valves close to the houses. When a given volume of wastewater is collected in the chamber sump, a pneumatic controller is activated by hydrostatic pressure. The controller opens an interface valvefor an adjustable time period. The wastewater (10 to 50 L) and a certain amount of air (20 to 60 L) are evacuated through the open valve into the vacuum sewer line. The pressure gradient between the vacuum station and atmospheric pressure at the collection pits is responsible for the movement of sewage to the vacuum tank (GTZ 2005).

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Vacuum Station

 

Small vacuum station building with biofilter for the suction air and collection tank. Source: ROEDIGER (2007)

All the vacuum sewers are connected to the vacuumcollection vessel installed at the central vacuum station, where vacuum pumps create the required negative pressure (approximate -0.6 bar). The vacuum vessel can be placed inside or buried outside the vacuum station. Transfer pumps convey thewastewater from the vessel to the wastewater treatmentplant or to an existing sewer. The capacity and dimensions of the vacuum station are dictated by the particular requirements of the sewer system. Operation of the vacuum and transfer pumps is controlled by a software (ROEDIGER 2007).

Piping

In contrast to gravity sewer pipes, it is easier and cheaper to build the vacuum sewer piping. Nomanholes or sewer pumping stations are necessary, just inspection points for pressure testing. This avoids settling of sludge and no manholes are needed to be cleaned out (see also dignity orhuman powered emptying). The pipes of a vacuum sewer system have a smaller diameter (80 to 250 mm) and the trenches are narrow and shallow (1.0 to 1.2 m). That is also an advantage if there is a high groundwater table. Unexpected obstacle can easily be bypassed by a modified and more flexible pipe design. If a pipe is damaged, the risk of sewage infiltration is very low, because of the negative pressure in the sewer line. Expert design is required, but the construction and installation work can be done by local contractors and pipe suppliers. No heavy machinery is necessary and there is no danger of a collapse of deep trenches (ROEDIGER 2007).

            

Local workers at construction of a shallow vacuum sewer system (above) and excavation of gravitysewer trenches with heavy machinery. Source: ROEDIGER (2007)            

Costs Considerations

As it is a high-tech system, it is costly. But in comparison with a conventional sewer system, it is cheaper. Piping costs are lower, because the dimensions are smaller. Therefore, less material is required. Also the installation can be cheaper because piping is independent from the topography. Furthermore, no heavy machinery is necessary for excavate deep trenches, thus the work can be done by local workers, which creates employment. Finally, big amounts of flushing water can be saved which is economical and ecological reasonable. However, the constantenergy requirement for the permanent vacuum generation can increase the costs.

Operation and Maintenance

The risk of clogging is low and there is almost no cleaning/emptying work to do. From time to time pressure in the vacuum sewer system should be tested. The system needs instructed workers for maintenance and operation works. More complex and/or technical problems are in the responsibility of the manufacturer.

                 

Construction of a sewer at the left side and pressure testing et the right side. Source: ROEDIGER (2007)                 

Health Aspects

As long as it is maintained properly, a vacuum sewer system guarantees a high level of comfort and hygiene. There is a very low risk of contamination due to leakage. It is a closed system, thus there is almost no contact between wastewater and operators. However, a treatment system at the end of the pipe needs to be in place.

At a Glance

Working Principle	Vacuum sewerage systems use a central vacuum source to convey sewagefrom individual households to a central collection station.
Capacity/Adequacy	Can be used in dense populated areas as well as in rural areas. It is independent from the topography (hilly or flat area) and can pass any obstacles without any problems
Performance	Very high performance
Costs	High capital costs, but still lower than gravity sewer system
Self-help Compatibility	Very low
O&M	It is a reliable system and does not need a lot of maintenance
Reliability	Very reliable
Main strength	Shallow trenches and it requires a minimal amount of flushing water. Very high level of comfort and hygiene.
Main weakness	It is costly and it needs a permanent energy source for the vacuumpumps. It needs expert design and depends on a centralised system.

Applicability

Basically, vacuum sewering is most suitable in areas where a collection is needed but other options are too costly or not feasible (Adapted from HUBA & PANZERBIETER 2006);

• Flat topography: gravity systems demand installation at great depths to maintain adequate flow (pump stations, lift stations)
• Rock layers, running sand or a high groundwater table make deep excavation difficult
• Areas short of water supply or poor communities that must pay for and cannot afford great amounts of water necessary for operation of gravity systems
• Areas that are ecologically sensitive
• Areas where flooding can occur
• Areas with obstacles to a gravity sewer route
• Installation of a new fresh water network, allowing sewerage pipe installation in the same trench

Where potable water is in short supply and/or people are poor, flushing velocities in gravitysewers are often difficult to attain and maintain. A vacuum system relies on the negative pressure to propel the liquid at scouring velocities and it is largely independent of the volumes of water used.

Advantages

• Requires less water to transport the excreta and faeces to the centralised treatment system
• Considerable savings in construction costs, and much shorter construction period
• Piping: Pipelines laid in shallow and narrow trenches; small diameter pipelines, flexible pipeline construction, easy to lay pipelines around obstacles
• Sewers and water mains can be laid in a common trench
• Closed systems with no leakage or smell
• No manholes along the vacuum sewers
• One central vacuum station can replace several pumping stations

Disadvantages

• Needs expert design
• Needs energy to create the permanent vacuum
• Relatively high capital costs
• Recycling of nutrients and energy becomes difficult
• Unsuitability for self-help, requires skilled engineers operators
• It is still a flushing system which transports wastewater away. If there is no treatment plant and an unprofessional discharge it can contaminate the environment

References 

GTZ (Editor) (2005): Vacuum Technology (Low Pressure Systems). Eschborn: German Agency for Technical Cooperation (GTZ) GmbH. PDF

HUBA-MANG, E.; PANZERBIETER, T. (2006): Sanitation is more than Life - Sustainable Sanitation Options for Sri Lanka. Singapore: World Toilet Organization (WTO). PDF

PDH ENGINEER (Editor) (2007): Vacuum Sewers: Design and Installation Guidelines. Alexandria, Virginia: Water Environment Federation. URL [Accessed: 09.03.2011]. PDF

ROEDIGER (Editor) (2007): RoeVac Vacuum Sewer System. Hanau: Roediger Vacuum GmbH. URL[Accessed: 09.03.2011]. PDF

UNEP (Editor) (2002): A Directory of Environmentally Sound Technologies for the Integrated Management of Solid, Liquid and Hazardous Waste for Small Island Developing States (SIDS) in the Pacific Region. The Hague: United Nations Environment Programme (UNEP). URL [Accessed: 28.03.2012]. PDF

ROEDIGER (Editor) (2007): RoeVac Vacuum Sewer System. PDF Presentation. Hanau: Roediger Vacuum GmbH. PDF

How to use the Report Feature of the HGL Plot in InfoSWMM

Subject:   How to use the Report Feature of the HGL Plot in InfoSWMM

The report feature of the HGL plot helps you understand in more detail the pump flows, forcemain flows and node heads.

Step 1. Load the Domain in the HGL Plot using Report Manager

Step 2. Click on the Report Command to Show the HGL Data in Tabular Format

Step 3.  Format the Results Table from the HGL Plot to see the data better.

Step 4.  Now we have the heads, flows and velocities for the pumps, nodes and force main links in our Domain around the pump of interest at time steps of 2 seconds,  We can now see how the flows, heads and velocities change downstream from the pump.

Step 5.  Force Mains, Nodes and Pumps in our Table

Step 6.  The pump turns on and the flow moves downstream to the force mains – the heads in the nodes increase to balance the flow at each node.  As you can see there is a 1 to 2 GPM decrease due to attenuation as the flow from the pump moves into the force mains.

Step 7.  The pump turns off and flows downstream decrease.  You can get negative flow if the downstream head is higher than the upstream head of the link.

Step 8.  Use Advanced Labeling and the HGL Plot Stepping Interval to see all of the data in your Plot.

How to Set Up Hot Start Files in InfoSWMM for a Fixed Boundary Outfall

Subject:  How to Set Up Hot Start Files in InfoSWMM for a Fixed Boundary Outfall

If you have a fixed boundary outfall condition in your model and want to prevent reverse flow when you run your simulation the best way is to use the Hot Start files to fill up the links and nodes at the start of the simulation.

1^st Step:  Turn off the DWF inflow so that ONLY the flow from outfall enters the network.  Use the Process Models in Run Manager to turn off and turn on the Dry Weather flow.

2nd Step:  Run the simulation first SAVING a Hot Start File using zero initial node depths and link flows.

3rd Step:  Save and Use Hot Start Files until the initial and final volume in your Network stays the same. 

4th Step:  Check the Initial and Final Stored Volume in the output text file

5th Step:  Check to see if you nodes are stable by using a Junction Group Graph

6th Step:  Now Run the Simulation with flows turned an and the network will start out with the Boundary Condition depths and stable flows

3 Types of Subcatchment Flow in SWMM 5

Subject:   3 Types of Subcatchment Flow in SWMM 5

There are three types of Subcatchment flow in SWMM 5

 1.   Impervious area with depression storage in which the runoff from the precipitation is delayed due to the depression storage.  Evaporation occurs based on the depth of water in the subarea of the Subcatchment.

2.   Impervious area without depression storage in which the runoff from precipitation is NOT delayed.  Evaporation does occur based on the depth of water in the subarea of the Subcatchment.
3.   Pervious area with depression storage in which the runoff from the precipitation is delayed due to the depression storage.  Evaporation and Infiltration occurs based on the depth of water in the subarea of theSubcatchment. 

How to Set a Flap Gate in InfoSWMM

Note:  How to Set a Flap Gate in InfoSWMM

You can set a flap gate in InfoSWMM either by using the attribute browser and changing the Flap Gate Installed to Yes or No or by using the DB Editor and changing the Flag for Flap Gate Installed to Yes by using the Block Edit tool.   The DB method is better for changing many conduits. 

SWMM 5 Arrow Direction Question

Subject:   SWMM 5 Arrow Direction Question

A question often asked is what happens to the flow in a SWMM 5 link if the downstream and upstream node names are entered in reverse.    The flow will be exactly the same as if the nodes were entered in the right order but the flow in the link will be negative.  As long as the inflow to the model is at the same node in both alternate models the node depth, the link upstream depths and upstream cross sectional areas will be the same.  For example, the flow out of Nodes 80408A and 80408 will be the same but the flow out of 80408 will be negative and the flow out of 80408A will be positive.

SWMM 5, H2OMap SWMM and InfoSWMM Time Step Guide

Subject:    SWMM 5, H2OMap SWMM and InfoSWMM Time Step Guide

If you use a variable time step in SWMM 5 or InfoSWMM/H2OMAP SWMM it is hard to gauge the proper value of the conduit lengthening.  You want to use a value that does not increase the volume of the network yet does increase the length of the shortest links so you can use a longer time step.  A good approximation to the time step that you want to use is shown in the image.  

The Time Step Guide in seconds is Link Length / [Velocity + sqrt(g*Maximum Depth)] with the assumption that the velocity at maximum depth is about the value of the wave celerity for closed links or sqrt(g*Maximum Depth).  Normally (unless pumps are involved) the average time step used during the simulation is a good gauge of the time to use for the simulation.  For example, in this model run the time step used is 13 seconds which is about the conduit lengthening time step of 20 seconds * adjustment factor of 0.75

Siphon Simulation in SWMM 5 and InfoSWMM

Subject:  Siphon Simulation in SWMM 5 and InfoSWMM

A Siphon is simulated in SWMM 5 and InfoSWMM using the basic node and link data and downstream boundary condition:

1.   Inflow can be time series, dry weather flow pattern, wet weather inflow or Subcatchment Runoff,
2.   The boundary condition can be either a free outfall, fixed or time series,
3.   The node invert, node maximum depth and node surcharge depth are defined by the user or network,
4.   The link lengths, diameters, link offset depths upstream and downstream are defined by the user of the network,
5.   The node depths, link flows, link depths and link cross sectional areas are calculated at each time based on the node continuity equation and the link momentum and continuity equation.  The link flows are a function of the friction loss, head difference across the link and the difference in the cross sectional areas of the link.
6.   In the particular model the Inflow at node MH1 fills up the MH1 depth which causes the links downstream to start flowing – the head difference across the links drives the flow up and over the siphon.

Aquifers in SWMM 5

Subject:   Aquifers in SWMM 5

Groundwater in SWMM 5 is modeled as two zones: (1) Saturated and (2) Unstaturated.  The data for the Groundwater Simulation consists of physical data in an Aquifer and elevation and flow coefficient and exponent data in the GroundwaterData.  The Aquifer data object can be applied to multiple Subcatchments but each Subcatchment has its own set of Groundwaterdata.  For example, in this model all of the Subcatchments share the same Aquifer data but each Subcatchment has different elevation and flow data – the labels on the basin are the groundwater elevations.

InfoSWMM and H2OMap SWMM Pump Summary Table

Subject:   InfoSWMM and H2OMap SWMM Pump Summary Table

The Pump Summary Table in Report Manager tells you how often the pumps turn on (Start-Up Count), the percent of the simulation time it was used (Percent Utilized) and the maximum, minimum and average flow for the pumps.

You can also see flows in the downstream links from the pumps in the force mains along with the pumps.

 

If you use the Mixed Graph Control you see the Pump flows and Link Flows on the same Graph

You can control the replay of the HGL Plot by altering the stepping time in Graph Settings