When alerts are acknowledged, present indication of "Alert Ack" on the active alerts list, maintain acknowledgement history and provide quick list of acknowledged alerts as we have with "ALL UNACKNOWLEDGED ALERTS" today.
Alert Ack could be included as another column as acknowledgement date/time. Ack date/time could be linked to acknowledgement history. A list of "Acknowledged Alerts" on a details page could alert another viewer that someone else is aware, working on it, etc.