---
title: "Coliforms | Definitions"
slug: "coliforms"
description: "Read about coliforms and relevant equations for the 1D water quality solver. "
tags: ["1D river modelling"]
updated: 2022-08-05T05:12:05Z
published: 2022-08-05T05:12:05Z
canonical: "help.floodmodeller.com/coliforms"
stale: true
---

> ## Documentation Index
> Fetch the complete documentation index at: https://help.floodmodeller.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Coliforms

The unit models transport and decay of coliforms in a river.

## Description

The variable representing coliforms is simply a transported substance that can decay according to first order kinetics. It does not interact with any other variable. It therefore behaves in all respects like a decaying pollutant (see DECAY).

The primary use is to model the variation of E-coli or any other bacteria. The only parameter required to model them is their decay rate expressed in terms of a T90 value. T90 (in hours) is the time taken for a population to reduce to 10% of its original density. In this respect it is different from DECAY which uses a decay rate (K) instead of T90. The T90 value can be varied between daytime and night-time. This allows for the increased mortality of the bacteria due to the effect of sunlight.

## Equations

The decay of coliforms is simulated by a first order decay equation:

![image](https://cdn.document360.io/5074bc26-a9ef-4bf4-b1de-a5c6ce628aca/Images/Documentation/image136.gif) (1)

Where: K = decay rate (s-1) C = concentration of coliforms (user defined units)

The first order decay rate constant, K (sec-1) is obtained from T90(hours) by:

![image](https://cdn.document360.io/5074bc26-a9ef-4bf4-b1de-a5c6ce628aca/Images/Documentation/image138.gif) (2)

## General

The COLIFORMS module may be run independently of the other water quality modules. The COLIFORMS module simulates the transported variable name:

- Coliforms (user-defined units)