Improvements in the Wastewater Flows Calculation in CLOACAS (SEWERS) 2.0 Version

The current version of CLOACAS (SEWERS), our sanitary sewer design software, incorporates new features regarding the wastewater design flows determination, since we have incorporated the spatial allocation and potable water demands calculation engine from our other civil engineering software: The Water Demands Allocator.

The object, “Water Demand Parcel,” has been included, which is a point that exists in the study area with certain characteristics in terms of water consumption, and which can be automatically allocated to a sewer pipe section (depending on its proximity to it) in order to estimate for each pipe section the total affluent water flow as the sum of all the allocated parcels’ water demand.

You will find in our sanitary sewer design software’s ribbon, a tab called PARCELS and, as you will see in this picture…:


…this tab incorporates the necessary tools for the definition and allocation of not only the Demand Parcels but also of the Demand Polygons which we introduced in our software for Water Network’s Demands Allocation.

This video explains how to create those objects, either by drawing them directly in the software’s drawing area or by importing graphic objects (text and polylines) in a drawing file with AutoCAD’S DWG format.

Then you will see that, with this incorporation, options or possible variations for the wastewater design flows calculation, which you can use in your sanitary sewer networks projects, are multiple, and we wish to give you ideas of possible combinations below:

Combining Water Demand Parcels and Polygons for Calculating the Wastewater Design Flows

The Wastewater Design Flow determination in an urban sanitation project depends, in most cases, on the potable water demand calculation for the sector under study.

The calculation of the potable water demands depends, on the one hand, on knowing the type and use characteristics of each building (or in general, of each lot or parcel within the studied urban area) and, secondly, on the usage factors, usually normative, that indicate what the minimum water consumption requirements are for residents to meet their health needs in these buildings.

When you face the design of a sanitary sewer system, you could have the following situations regarding the available information:

1. It is known, in detail, each building’s (or lot’s) characteristics of use in the study area.

This is the most common condition for new housing complexes which have not yet been developed and in which the design starts from an urban plan where a trained professional in the subject (urban planner) has made the appropriate subdivision of developable areas, assigning for each of them a land use where it is specified, for example, for single family homes, or a lot area or, in the case of multifamily housing, the number of stories, apartments, and bedrooms, among other factors.

Here, the wastewater design flows calculation with CLOACAS would involve defining one or more “Typical” Demand Parcels (watch this video), depending on the different types of building uses in the study area. Usually, unless the land use in the urban areas of interest is very heterogeneous, no more than two or three Typical Demand Parcels are required.

Once those Typical Demand Parcels are created in a sanitary sewer network project, you just have to insert them (in the software’s drawing area, either drawing them one by one, or by importing from a drawing file), usually in what would be the front of each lot so that the software performs the automatic spatial allocation to the nearest sewer pipe section in the sanitary network you are about to design.

In the Settings panel at the software’s PARCELS tab, you can enter a maximum distance for Demand Parcels’ assignment, so that you can control the process of water demands calculation by limiting those parcels that are too far from a sewer pipe may be allocated to it erroneously.


In case a sewer pipe section within the specified length is not detected, the software will warn you, after you click the “Assign Demands” button, that some Demand Parcels remain as “unassigned” through a message like this:


In any case, after performing the automatic allocation, the summary calculation table will be shown. In this table are listed the total number of visible parcels in the drawing area; the number of those that have been assigned (which are within the distance previously specified); and the respective water demands allocated:


Note that the difference between the numbers presented in columns 2 and 3 of the table tells you that there are demand parcels that have not been assigned to any sewer pipe section, so as to warn you that the necessary modifications should be done (usually you only need to slightly change its location on the drawing area so that it is close to any sewer pipe section) in order to include them in the wastewater design flows calculation.

When you perform the water demands calculation (by clicking the Assign Demands button at the PARCELS tab), you will see in the table of parcels (and polygons, if appropriate), quantity, and demands allocated to each sewer pipe section in your sanitary network’s project:


Those average potable water demand values will be the starting point to determining the respective wastewater design flows within the software, as explained in this tutorial.

2. Characteristics of the Land Uses are known within the study area.

Here we refer to situations such as:

2.1. The population density (inhabitants per hectare, for example) is known in the study area.

When urban development plans for the sector you are studying are available, it is common to find maps or drawings where several “land use polygons” are drawn in order to define the intended use of the urban areas.

You could well have polygons within which it is defined that the maximum allowed population density, for a residential area is, for example, 150 inhabitants per gross hectare. This extension is defined, of course, by the measured area in the reference map for the respective polygon.

Here, implicitly, you have the value of the saturation’s population of all sectors within the polygon in question with which, by establishing a per capita water demand, you may obtain the corresponding total potable water demand.

In this case, you should then create the Demand Polygons of the “Population Density Type,” as explained in this video.

A variant of this condition is that you set, for your sanitary sewer network’s project, a design population with which you can calculate a population density by dividing its value between the area (net or gross) of the studied area.

2.2. You do not know the population density, but you know usage characteristics within the polygons

In some cases, for example in commercial or industrial areas, urban development plans do not set a specific population density, as it is assumed that this value is associated only with residential usage of the land.

That’s when global water demands are used to calculate the corresponding demand for these sectors.

These global water demands are expressed as unitary flows (flow between area).

So you could have, depending on what standards and regulations are specified in your country, unitary flows (or unitary demands) of, for example, 0.5 liters per second per hectare for commercial areas or 1 l/s/ha for industrial areas.

Thus, if you know the unitary demand as well the extents area of the corresponding polygon, the respective demand is easily obtained through this two values product.

When you have this kind of situation in CLOACAS, you may then use Demand Polygons of “Unitary Flow Type,” for which it is necessary to specify the corresponding unitary flow in the Water Demand Polygons dialog:


In this tutorial created for our Water Demands Allocator Software, you will know the available types of demand polygons as well as the details to take into account when you are creating one of them.

What should I consider when I use Demand Polygons in a sanitary sewer network project?

There are several aspects that we want to make sure you know when you’re using demand polygons in projects created with our sanitary sewer design software:

There is not a maximum assignment length for Demand Polygons.

The process of automatic assignment of calculated water demand for any polygon in your project begins with determining which is the closest sewer pipe section to the visible polygon’s centroid but, unlike with Demand Parcels, there is no verification for a maximum allocation distance.

Thus, when you perform the parcels and polygons demand allocation, clicking the “Assign Demands” button, all polygons will be assigned to a sanitary pipe section.

Here you could find cases where you will clearly see in the drawing area that the allocation is not adequate; for example, look at this picture:


The polygon’s assignment line (the thin light gray line) indicates that the water demand calculated for this polygon has been assigned to a sewer pipe section that is not the rightful one by proximity, but it is the one for which was detected the shortest perpendicular length to it (from its centroid’s location) among the other pipe sections within the sanitary sewer network studied.

Certainly, here the water demand calculated for this polygon should be associated to the A1.3-A1.4 sewer pipe section, but how do you control this in CLOACAS?

The way to solve situations like this is to edit the polygon (select it, right click the mouse, and click on the Properties submenu):


There you will see that, besides the information about water demand calculations for the edited polygon, you have the Manually Assign to Sewer Pipe checkbox, which lets you override the automatic assignment performed by the software, letting you select from the list, the pipe section to which this water demand polygon will be affluent.

When you make the change and close the properties dialog, again, click on the Assign Demands button; you see now that the assignment line changes:


It is indicating that the allocation is the appropriate.

Water Demands Assignment summary tables will include the results for the Demand Polygons.

Needless to say, when you include Demand Polygons in the Wastewater Flows Calculation in CLOACAS, summary tables will show the corresponding information. For example, in this picture you see the result when you have a sanitary sewer network project with Parcels and Polygons visible in the drawing area:


And, if you choose to see the sewer pipe table from the list of available tables, you will see that the Demand Parcels and Polygons assigned to each section of the sanitary sewer system is also indicated:


So, you see, the options you have to perform the wastewater design flows in your sanitary sewer network projects are multiple when you incorporate the demand parcels and polygons objects.

Remember that, for the purposes of the water demands calculation with this option, CLOACAS only takes into account the parcels and polygons that are visible at the time of pressing the Assign Demands and Calculation buttons in the PARCELS and SEWER PIPES tabs, respectively.

This visibility is controlled through the respective layers, in the Project Layers dialog:


This last option allows you to perform the calculation and design for different developing stages of the urban sector, which could be, for example, to use a Demand Polygon type to represent future development areas (or, also, to use demand parcels) to which, once created in the drawing area, you turn off the visibility of the respective layer so they´re not included in the calculations at the first stage analysis.

Quite versatile, right?