Call this at the top of Application_Start. Any key/value pairs (delimited by '=') will be loaded into the processes environment.

EnvironmentVariables.cs

  public static class EnvironmentVariables {
    public static void SetUp() {
      var configFile = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "..", "Environment.config");

      if(File.Exists(configFile)) {
        var lines = File.ReadAllLines(configFile);

        foreach(var line in lines) {
          var index = line.IndexOf("=", StringComparison.Ordinal);

          if(index > -1) {
            var key = line.Substring(0, index);

            if(key.Trim() != "") {
              var value = line.Substring(index + 1);
              Environment.SetEnvironmentVariable(key, value, EnvironmentVariableTarget.Process);
            }
          }
        }
      }
    }
  }

See: https://gist.github.com/jwatney/c00ac732f2d3ebec58801e7ff17a8408