Pulp and paper mill boiler feedwater needs to be of high purity to maintain safe, reliable boiler operation. When feedwater contamination occurs, it may cause either high or low boiler water pH excursions, steam contamination, or insulating deposits. Common boiler feedwater
contaminants are silica, black liquor, hardness, and iron. For each of these contaminants, this paper discusses sources of contamination, potential problems caused by each particular contaminant, and recommended actions to be taken when contamination occurs.