Code to modify the web.config in SP system

gistfile1.cs

var modifications = new List<SPWebConfigModification>()
                        {
                                new SPWebConfigModification("remove[@name='BotDetectCaptachHandler'",
                                                            "configuration/system.webServer/handlers")
                                {
                                    Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode,
                                    Value = @"<remove name='BotDetectCaptchaHandler'/>"
                                },
                                new SPWebConfigModification("add[@name='BotDetectCaptchaHandler'",
                                                            "configuration/system.webServer/handlers")
                                {
                                    Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode,
                                    Value = @"<add name='BotDetectCaptchaHandler' preCondition='integratedMode' verb='GET' path='BotDetectCaptcha.ashx' type='BotDetect.Web.CaptchaHandler, BotDetect' />"
                                }
                        };

            SPSecurity.RunWithElevatedPrivileges( () =>
                                                      {
                                                          var contentService = SPWebService.ContentService;
                                                          foreach (var modification in modifications)
                                                          {
                                                              contentService.WebConfigModifications.Add(modification);
                                                          }

                                                          contentService.Update();
                                                          contentService.ApplyWebConfigModifications();
                                                      });
            

This is included in the WebApplication scoped FeatureReceiver under FeatureInstalled event handler.