Part 6 of 8: Nutrients
Microorganisms consume organic matter to get the energy they need for cell maintenance and reproduction. The bio chemical work done by the cell is called metabolism. Some of the molecules are broken down to provide energy. Others are used as building blocks in the manufacture of new cell parts, a process called synthesis. When a cell has enough energy and the necessary building blocks, it reproduces, thus sustaining its life. For this process to occur continuously and at a fast rate, the microorganisms must have a balanced diet, just like humans. The major chemical elements for growth are carbon (C), oxygen (O), nitrogen (N), and phosphorus (P). In the context of an Aerated Stabilization Basin (ASB) system, the carbon comes from the Biochemical Oxygen Demand (BOD) in the mill effluent, the oxygen from the ASB aeration system, and the nitrogen and phosphorus from the nutrient addition system and benthic feedback. Nitrogen and phosphorus are collectively referred to as macronutrients, or simply nutrients, in the context of wastewater treatment.
While it is possible for bacteria to consume BOD without nutrients, the complete synthesis of new cells requires nitrogen and phosphorus in the proper amounts. Without suffi cient numbers of bacteria, an ASB cannot achieve optimum BOD removal. This is because there is no mechanism for increasing the biomass via sludge return, or recirculation, as is the case with activated sludge. Other problems caused by insufficient nutrient levels are well documented in the literature and include exces sive filamentous organisms and polysaccharide or slime bulking.
Nitrogen is available to the biomass as either ammonium (NH4+) or nitrate (NO3–), with ammonium being the preferred source. The organic removal rate is higher when ammonia is the primary nitrogen source. Phosphorus must be in the form of soluble orthophosphate (o-PO4) in order to be assimilated by the biomass.
The theoretical amount of nitrogen and phosphorus re quired for new cell synthesis is dependent on the growth condi tions of the system. The rule of thumb is to provide an influent ratio of 100:5:1 (BOD:N:P). The actual nutrient requirement will depend on the net biomass synthesis. This balance relates to the nitrogen and phosphorus content of the wasted sludge as determined by the wastewater characteristics and mean cell retention time (MCRT). For ASBs, the required ratio is generally less than one-half that of an activated sludge system. This is primarily due to significant amounts of nutrient recycle from the benthic (anaerobic) activity in unaerated sections of ASBs.
Nutrient control and optimization in aerated stabilization basins is a critical component in maintaining wastewater permit compliance. Environmental Business Specialists is a leader in providing customized nutrient blends to the pulp and paper industry with our MacroGro nutrient blends. In addition to selecting and providing the correct formulation, we utilize respirometry and BOD surrogate development to assist our clients in achieving the best possible system performance at the lowest overall cost of operation.
For more information on EBS MacroGro nutrient blends and our support tools, contact us at firstname.lastname@example.org.
This article is adapted from “Aerated Stabilization Basins in the Pulp and Paper Industry” by Paul Klopping and Mike Foster. Copyright 2003. Callan & Brooks. For information on purchasing a copy of this manual, contact EBS.