RAS Chlorination for Filamentous Bacteria Control (Essentials)
There are no general 1 size fits all solutions for troubleshooting biological wastewater treatment system
issues and while it is always preferred to address the root source of a problem, there are simply times in
which this may not be feasible, or logistically practical. A common instance of these scenarios includes
troubleshooting challenges with filamentous bacteria bulking.
Filamentous Bulking Defined
Textbook definition generally defines filamentous bulking as conditions in which the mixed liquor
possesses an SVI (sludge volume index) value of >150 mL/g. This definition often generally applies to
many systems, however the actual SVI values in which clarifier failure is reached (as solids entering the
clarifier at a higher rate than can be removed with corresponding suspended solids carry-over) has many
significant factors such as the size of the clarifier (most specifically the surface area) and the hydraulic
flow rate. For example, there may be treatment plants that are already near or beyond their design
hydraulic capacity and solids loss from the final clarifier (s) may be reached at significantly lesser SVI
values than 150 mL/g in these instances. Vice versa, in situations where the plant is well within range of
its hydraulic capacity and if the clarifier (s) are large enough, higher SVI values may be possible while still
maintaining a low sludge blanket and optimal effluent treatment parameters.
It is critical to be able to determine the differences within each system and determine on a scale of 1-10
how severe, or how much risk a potential scenario poses on clarifier performance. In systems such as
municipal systems with high I and I (inflow and infiltration) potential, these large increases of flowrate
must often be taken into consideration in context of how saturated the ground is, and how likely events
such as a heavy rain of a snow melt are to significantly impact the hydraulic flow rate.
RAS chlorination is the process of specifically targeting undesirable filamentous bacteria with
disinfectant (generally sodium hypochlorite) to systematically kill filamentous bacteria to improve SVI
values, while reducing damage to bacteria within the floc as much as possible. In addition to risk
assessment, the choice of RAS chlorination should also be compared to other methods such as sludge
juggling (adding additional basins online etc.), chemical settling aids, and others. RAS chlorination is
generally most successful when flocs are strong and there are high amounts of filamentous extending
from or bridging the flocs together. Microscopic evaluation is important to determine the strength of
the floc structure, the location of the filaments, the overall health of the biomass, and ideally the rank
and abundance of the filamentous bacteria morphotypes that are present in order to assist with
changes in the wasting rate during the period of RAS chlorination.
General Dosing Guidelines
It is important to remember that each system is different, and the chlorine dose required for targeted
kill of filamentous bacteria varies depending upon the chlorine demand brought upon by factors such as
competing reactions. Generally, municipal systems are able to dose much closer to general guidelines,
while various industrial wastewater processes may require significantly higher amounts of chlorine.
Remember to start conservatively with chlorination as the dose can easily be increased, while if the
chlorine is applied to aggressively, this can cause many negative impacts to the health and settling
characteristics of the biomass. It is common for “light” RAS chlorination to involve addition of 2-3 lbs. of
active chlorine per 1000 lbs. MLVSS of the mixed liquor in the aeration basin. Moderate dosages
generally range between 5-6 lbs./1000lbs. MLVSS, and what would be considered generally higher
dosages are >8 lbs./1000lbs. MLVSS.
It is worth mentioning that in rare instances (such as lagoons that are converted to large extended
aeration activated sludge systems) that in these instances we prefer to chlorinate only the estimated lbs.
of MLVSS expected to pass through the clarifier within a 24-hour period. Note that RAS chlorination is
most effective when each “bug” comes into contact with chlorine 1-2x per day minimum. In instances in
which the 1-2x chlorine to “bug” exposure may not be possible options include multiple chlorination
addition points, a hypochlorite “bomb” (which due to high risk is only offered through consulting) or
understanding that a significantly longer period of time may be needed for any potential success (along
with the increased potential that chlorination may not be frequent enough to see beneficial results).
When viewing aerobic biological flocs under fluorescent microscopy, it is common for the majority of
the bacteria within the flocs to be dead or non-viable and the majority of the filaments (often 95% or
greater) to be healthy/viable. In most scenarios, a decrease in the wasting rate is warranted (i.e., 10% or
more) when RAS chlorination is applied due to the fact that filamentous bacteria are extremely efficient at oxidizing/treating carbonaceous organic material (BOD).
Application and Monitoring
It is essential that chlorine is added to the RAS in an area of high mixing. Most often these locations
include areas such as the RAS piping or the center well of the clarifier. The addition of chlorine to locations of poor mixing (such as RAS wet wells) does not allow for even distribution of the chlorine. When calculating the RAS chlorine dose, it is important to adjust the calculations to include the chlorine source (i.e., sodium hypochlorite) to 100% “purity”. The type (s) of filament that is being chlorinated (sheathed versus non-sheathed) have a significant impact in the time needed for improvement in the settleability as empty sheaths with dead filaments often still negatively impact SVI values. When chlorinating the RAS, daily microscopy with phase contrast oil immersion 1000x is desired to monitor the health of filaments. Once 50-60% of filaments show signs such as damaged cells or empty sheaths, the RAS chlorine is generally reduced slightly to prevent over-chlorination.
Chlorination for filamentous bacteria control is only a “band-aid” option and if the conditions remain in
which filaments outcompete floc forming bacteria, it is common for filaments to grow back rapidly once
chlorination is ceased. Some systems use light RAS chlorine “maintenance doses” to help offset these
challenges, however, this is case specific and depends upon other options available, economic logistics,
availability of operational personnel etc.