March 2013, Vol. 25, No.3

Operator Essentials

What every operator should know about chemically enhanced primary treatment

Henryk Melcer

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Knowledge  

Principle  

Practical considerations  

Combined sewer overflow (CSO)  

 

Release that can occur from combined sewer systems during periods of heavy rainfall or snowmelt.  

 

Combined sewer systems historically were designed to collect rainwater runoff, as well as domestic and industrial wastewater, in the same pipe. These systems transport all of their wastewater to water resource recovery facilities (WRRFs). 

  

During heavy rainfall or snowmelt, wastewater volumes can exceed the capacity of the sewer system or treatment facility. To relieve this situation, combined sewers were designed with overflows that occasionally discharge excess wastewater directly to nearby waters. 

  

CSOs contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris.  

 

Sanitary sewer overflow (SSO)  

 

Release from sewers designed to carry wastewater alone when those sewers are overwhelmed by infiltration of rainwater.  

 

Infiltration and inflow enter collection systems through defects that occur as lines age (infiltration) or through inappropriate connections of storm drainage into the sanitary collection system (inflow). 

  

Older sewer design standards often permitted sources of stormwater into sanitary sewers in the form of roof and basement drains.  

 

Wet weather flows  

 

Wastewater flows occurring during rainfall or snowmelt composed of CSOs, SSOs, or bypasses around WRRFs.  

 

Regulations for each wet weather flow source require municipalities to develop management and control strategies.  

 

Chemically enhanced primary treatment (CEPT)  

 

The practice of adding chemicals to wet weather flows to encourage coagulation, flocculation, and settling of solids.  

 

In wet weather flows, influent wastewater particles are negatively charged and repel each other. Coagulants neutralize the charge and make it easier for the particles to gather together either naturally or with the aid of flocculants, thereby making the particles more readily settleable. 

  

Improved particle settleability can increase clarifier capacity by a factor of approximately 3. 

  

CEPT can be considered as an economically viable wet weather treatment alternative to proprietary high-rate clarifier systems for municipalities with existing primary clarifier facilities and achieve similar levels of performance with respect to total suspended solids (TSS; 75% to 95% removal) and biochemical oxygen demand (BOD; up to 65% removal).  

 

Coagulation  

 

The process by which charged particles are neutralized or colloids destabilized by the addition of coagulants, such as metal salts and some polymer materials.  

 

Common coagulants used in CEPT are ferric chloride, alum, polyaluminum chloride, and cationic polymers.  

  

Selection of the best coagulant is determined by jar-testing, compatibility with such wastewater properties as alkalinity, and cost. Typical dosages for alum and ferric chloride are 40 to 80 mg/L and 30 to 50 mg/L, respectively.  

 

Flocculation  

 

The process by which neutralized/destabilized particles join and form aggregates.  

 

Common flocculants used in CEPT are anionic polymers, typically with high molecular weights. 

  

Selection of the best flocculant is determined by jar-testing, compatibility with the selected coagulant, preference for physical form (powder or emulsion), and cost. 

  

Typical dosages are 0.2 to 2.0 mg/L as active polymer.  

 

CEPT applications  

 

CEPT may be used to increase primary clarifier capacity during wet weather events or to reduce secondary influent organic loading in lieu of adding more secondary treatment capacity.  

 

In wet weather cases, operation is intermittent, so CEPT facilities have to be maintained in a state of readiness to respond to storms. In capacity cases, CEPT is deployed continuously. Chemical storage and delivery capacities will be greater for this application than for handling intermittent wet weather flows.  

 

Settling velocity  

 

Settling velocity is proportional to particle velocity and particle density. This relationship is known as Stokes’ Law.  

 

The coagulation and flocculation processes result in making larger and denser flocs, which results in increased particle-settling velocity.  

 

Surface overflow rate  

 

This is the hydraulic capacity of a clarifier, measured as influent flow rate divided by clarifier surface area (gal/ft2•d or L/m2•d).  

 

Wastewater flow in a clarifier occurs upward against the downward movement of settling particles. Therefore, to increase hydraulic capacity (increase upflow velocity), it is necessary to increase settling velocity. Chemical enhancement increases settling velocity so the particles can continue to settle in a downward direction despite the higher upflow velocity of the influent. This enables the clarifiers to accommodate the higher flows associated with wet weather events.  

 

High-rate clarifier  

 

These primary clarifiers operate at elevated surface overflow rates — on the order of 20 to 40 times higher than standard primary clarifiers.  

 

High-rate clarifiers are vendor-supplied and can include pH adjustment, coagulation, flocculation and settling, and often lamella plates. These processes are optimized to provide treatment in minimum time, resulting in significant space saving. They can use more chemical than CEPT systems with correspondingly greater solids generation.  

 

Jar-testing  

 

This test protocol enables the simulation of coagulation and flocculation processes that occur in CEPT clarifiers.  

 

Banks of four or six 2-L square flasks are stirred simultaneously to simulate different levels of mixing. Chemicals are added to each flask to represent a range of doses. After an appropriate settling time, the supernatant is analyzed for TSS or turbidity to measure performance of the test chemical. To serve as a control, one flask is not dosed. Series of tests can be conducted to evaluate the performance of coagulants, flocculants, floc shearing effects, and hydraulic residence times. Jar tests often are conducted during a wet weather event to ensure that dosages are being applied correctly.  

 

Temperature  

 

Temperature influences optimal pH for coagulation and reaction times and wastewater viscosity.  

 

Optimum pH for alum use is 6.5 to 7.5; for ferric chloride, the range is wider: 4 to 11. Lower temperatures increase viscosity and reduce particle-settling rate.  

 

Alkalinity  

 

Alkalinity is required for coagulation. It is consumed at different rates according to the chemistry of a specific coagulant.  

 

Alkalinity consumption for alum is 5.5 mg alkalinity (as CaCO3) per milligram of Al fed and 2.67 mg alkalinity (as CaCO3) per milligram of Fe fed. Polyaluminum chloride compounds consume even lower levels of alkalinity.  

 

Dilution  

 

During wet weather events, the ingress of rainwater causes dilution of the influent wastewater.  

 

Rainwater contains no alkalinity and dilutes wastewater alkalinity. Because coagulants consume alkalinity, primary effluent pH can decline, which can compromise downstream nitrification (if deployed) or effluent pH compliance. Also, the dose of coagulant may change as dilution increases and influent TSS and BOD concentration decline correspondingly.  

 

Solids production  

 

Primary sludge production will increase according to the coagulant dose.  

 

The chemical solids produced during CEPT can increase primary sludge production by up to one-third.  

 

Nature of wet weather flows  

 

The intensity of rainfall and the period during which high flows enter a treatment plant are influenced by the attributes of a rainstorm.  

 

Rainfall intensity can change drastically from storm to storm. Summer storms can bring very high rain during a short period, resulting in rapid influent flow escalation that may cause CEPT to be deployed rapidly. Winter storms can last for several days in some parts of the U.S.; this brings steady rain that causes inflow to increase slowly but to stay elevated for long periods. Plant staff may choose to ramp up CEPT slowly; chemical consumption will be high if flows remain elevated for several days. Storms early in the fall may not result in high influent flows, because the ground is dry and may require several storms to become fully saturated and induce high influent flows. The number and magnitude of antecedent storms also affect influent flows.  

 

First flush  

 

This is the initial flow at the onset of a wet weather event.  

 

During early stages of a wet weather event, sewer flow velocity increases in response to the rain infiltration. This scours the collection system and resuspends materials that have settled during dry weather flows. The resuspended materials will elevate influent TSS and BOD for a period of 1 to 2 hours and induce a higher loading to the CEPT system. Chemical doses may have to be increased during this period.  

 

Henryk Melcer is senior process engineer in the Seattle office of Brown and Caldwell (Walnut Creek, Calif.).