May 2010, Vol. 22, No.5

Small Communities

Managing Activated Sludge Units in Small Decentralized Systems

Michael Hines and Jim Kreissl

Acceptance of decentralized sewerage systems to serve small communities has grown significantly during the last 20 years. As a result, the availability of proprietary and nonproprietary treatment technologies also has increased significantly. Treatment systems for individual residences, subdivisions, and small communities range from simple septic tanks and/or drain fields to complex mechanical and biological systems. Many small communities using the managed decentralized approach are served by treatment technologies virtually identical to those used at large municipalities.

The success of these various technologies depends on effective management of the design, construction, and operation of the system. The appropriate level of management increases with increasing complexity of the treatment system.

Small, community-scale decentralized systems using mechanical–biological treatment systems generally are permitted by regulatory agencies and require frequent monitoring, system visits, and careful operational oversight. Treatment units serving individual residences historically have used conventional septic tanks and/or drain fields with management limited to addressing system failures, which lead to liquid on the surface or backed up into building drains.

Increasingly, though, more complex biological–mechanical systems are being used at individual residences and other small systems. These units are either fixed-film or activated sludge systems. Activated sludge has been the treatment process of choice for many large American cities for nearly a century. With adequate operational management — which can be a challenge — the process does a good job of converting organics in domestic wastewater to carbon dioxide, water, and new microorganisms.

 

Defining ‘Adequate’

Adequate management, regardless of system size, includes controlling the organic and hydraulic loading rates, adjusting the aeration to match changing loading rates, removing excess activated sludge from the unit to prevent releasing solids into the discharge, and frequently inspecting and maintaining the aeration device to ensure its continuous operation. In large activated sludge systems, the frequency of minimum operational oversight ranges from continuous monitoring to daily or weekly inspections.

Frequency of management oversight for very small systems and systems serving individual residences should be at least monthly, and more frequent if necessitated by startup conditions, fluctuating loads, or malfunctioning equipment.

NSF International (NSF; Ann Arbor, Mich.), a nonprofit standards development and product certification company, created a protocol, NSF Standard 40, for certifying these small systems. Equipment manufacturers submit units to be tested under specified flow and organic load conditions. The units are operated with only limited operational adjustments for set periods of time. Units that meet the NSF Standard 40 effluent requirements can be marketed as NSF Standard-40-approved systems.

Many states have adopted regulations allowing small activated sludge systems to be used if the units have been certified under NSF Standard 40. However, real-world conditions vary so widely that few field installations are loaded and operated the same way that they are tested under NSF Standard 40. Additionally, inherent operational characteristics of the activated sludge process require operational management not considered or even allowed under the NSF criteria.

 

Improving Process Controls

For example, overaeration during periods of low flow (which equates to low food provided to the unit) results in an activated sludge that settles poorly, overflows the final settling step, and is discharged with the effluent to the drain field or subsequent processes. In larger onsite systems, timers or programmable logic controllers are installed to match aeration equipment cycles to the feed and starve cycles inherent in onsite systems. However, if a homeowner or the service provider attempts to add such timers to a small system, the system warranty likely could be voided, which also could be a regulatory violation.

NSF should revise its test protocol to allow for timer controls to be used on the aeration device. Regulatory and service provider groups should collect and provide data to NSF or other testing entities to develop a test protocol that more closely reflects real-world operating conditions.

Additionally, the activated sludge process works best when the ratio of food to microorganisms is maintained within an optimum range. In large plants, this is accomplished by controlling aeration, as well as the amounts of settled activated sludge returned to the aeration basin and sent on to solids processing (wasted).

Small systems do not have provisions for storing wasted sludge. Instead, the excess solids can only exit via the effluent to the drain field periodically. To remedy this, controls and waste-sludge storage tanks similar to those on large plants should be provided on the small systems. Once the unit has been in operation long enough to reach equilibrium, an operator trained by the equipment manufacturer should be onsite frequently to make adjustments to aeration cycles and sludge wasting. The frequency of such visits will vary from site to site, but they should occur at least weekly for the first few months and at least monthly thereafter.

Finally, the most critical component in the small activated sludge system — the aeration device — must be monitored. These may be small compressors or devices that aspirate air into the liquid using a propeller or a venturi fitting on a pump-discharge line. Operational oversight must be frequent enough to ensure that the aeration device is replaced as soon as possible upon failure. Some alarm mechanism should be required that alerts the resident or facility owner immediately upon failure.

 

System Ownership

In addition to refining process controls, who owns the unit can affect their efficiency. Now, homeowners or small business owners generally own the units. These individuals usually have little or no interest — nor any training or experience — in managing the operation of a biological treatment system.

Recognizing this, some states require owners to sign maintenance contracts with third-party service providers. These service providers are required to make periodic visits, usually semiannually or quarterly, to check operation and provide maintenance services.

To be truly effective, however, the frequency of visitation should be increased to no less than monthly (after the initial startup and adjustment period). Quarterly adjustments are not adequate to address the weekday, weekend, vacation, and heavy entertainment episodes that these small systems must handle. One other note: Even though homeowners often are required to have operational oversight, the homeowner, not the contractor, holds all compliance liability for any failure of the system.

To help ensure the proper level of operational management required by these systems, ownership or operational control should be limited to public utilities or other entities that have the needed experience. And, in such cases, system permits should hold these entities, rather than the homeowner, liable for performance.

The activated sludge process is the process of choice for large-scale wastewater treatment systems discharging to large waterbodies. In large systems with sophisticated operation and management, the process is capable of providing a high-quality effluent. With that same level of operation and management, the individual home and other small activated sludge systems should also be capable of providing good effluent quality. However, the cost to provide that level of management and the limited availability of properly trained operations personnel make reaching that level of treatment a challenge.

 

Michael Hines is founding principal at Southeast Environmental Engineering LLC (Concord, Tenn.), and Jim Kreissl is an environmental consultant based in Villa Hills, Ky.

 

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