5.3.3 Attached Growth Process + Construction Wetland/Stabilization Pond System

Categories: 5.3 Wastewater Treatment Process Design
Time of issue: 2022-04-28 18:37:52
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Description:

Information

A. Description of the system

The system consists of a septic tank, an attached growth process unit (TF or BRC), and a construction wetland or stabilization pond (Figure 5.5). System effluent complied with applicable standard is discharged to surface water. The attached growth process has a major advantage of lower energy consumption and easier operation compared to the activated sludge process. It is applicable for a cluster of houses and large communities.

In this system, the septic tank acts like a primary clarifier to remove settleable and floatable solids in the wastewater before it is applied to the attached growth process. For a large-scale system to treat wastewater from a large community, a clarifier is needed before the effluent from the TF or RBC flowing into the constructed wetland or stabilization pond to avoid excessive accumulation of solids and clogging. For a small-scale system for a cluster of houses, the clarifier is not necessarily due to the low suspended solid content in the effluent from the TF or RBC. The constructed wetland and stabilization pond are normally surrounded by a berm. Fencing will be installed on the berm, or immediately adjacent to the berm to prevent entry of animals and to provide security/safety.

The system relies on biological processes and continuous power supply, which cannot be sustained under seasonal use. It is not suitable for seasonal dwellings where water use and power supply are intermittent (e.g., rural vacation cottage).

Figure 5.5 Flow chart of attached growth and constructed wetland/stabilization pond process

B. Design Consideration

The septic tank, a primary treatment process, is necessary to remove settleable solids prior to the attached-growth process. Without proper removal, excessive solids will accumulate and clog the media causing organic overloading, anaerobic conditions, ponding, and fouling. The septic tank shall be well-designed to reduce organic loading and remove grease.
Recirculation is used to reduce the organic loading, improve sloughing, reduce odors, and reduce or eliminate filter fly or ponding problems. The amount of recirculation is dependent on the design of the treatment system and the operational requirements of the process. Typical recirculation ratios range from 1:1 to 4:1.
Flows can be recirculated from various points following the TF/RBC to various points before the filter. The most common form of recirculation removes flow from the TF/RBC effluent or clarifier and returns it to the influent of the TF/RBC.

Design of a septic tank, TF, RBC, constructed wetland or stabilization pond refers to Section 4.2.1, Section 4.3.1.1, Section 4.3.1.2, Section 4.3.4, and Section 4.3.3, respectively.

C. Construction and Commissioning/testing

Has signs located on each gate with a warning of “Wastewater Stabilization Pond – No Trespassing”, or “Constructed Wetland – No Trespassing”.
If the attached growth process is just being put into operation or if something has killed some of the slime microbes, the operator can recirculate filter effluent from before clarification. This flow will have some of the sloughed microbes in it and could be used to reseed the filter.
Before commissioning of a trickling filter, the operator shall: check pressure of air blower; check all air diffusers are working and adjust air valves to appropriate settings; check sludge and skimmer return lines are working; check float switch levels and pump activation levels; and check alarms work.

D. Operation management

Recirculation adds dissolved oxygen to the primary effluent to freshen the trickling filter influent. Recirculation rates at many facilities are controlled by the filter effluent dissolved oxygen (DO). If the DO drops below 1.5 mg/L, increase recirculation; if the DO goes above 2.0 mg/L, decrease recirculation.
The operator shall conduct daily inspection on the attached growth process unit to make sure the system is operating normally.
Preventing problems associated with odor, mosquito breeding, disease transmission by insects and exposure to animals are key considerations.