TM 5-814-3/AFM 88-11, Volume III
8-3. Techniques for upgrading existing trickling filter plants.
a. Upgrading to relieve organic and hydraulic overloading. Trickling filter plants may be upgraded
to relieve hydraulic or organic overloading by the construction of additional trickling filters in parallel with
existing units or by any one of the following four procedures:
(1) Conversion of low-rate trickling filters to high-rate tickling filters. The first step in this
procedure is to evaluate the quantity of recycled flow to be returned ahead of the filter. This can be
accomplished by using trickling filter formulae. Such an upgrading technique may require changing or
motorizing the existing distributor arm to handle the new hydraulic loading to the upgraded filters.
(2) Conversion from a single-stage to a two-stage filtration system. Low and high-rate trickling
filters that are overloaded can also be upgraded by converting them to two-stage filtration systems. This type
of upgrading, in which a complete set of units is added, is far less complicated than a renovation of existing
tankage. The most important consideration is that sufficient hydraulic head has to be available to operate the
individual unit processes.
(3) Conversion of single-stage trickling filter to two-stage biological system. An overloaded
high-rate trickling filter can be upgraded by replacing existing stone with synthetic media. Synthetic media
have been found to have the following advantages over conventional filters: higher allowable hydraulic
loadings; low energy consumption; reliable performance; resistance to hydraulic and organic shockloads;
simple operating procedures; and reduction in sludge bulk ing problems. Another upgrading technique is to
install a second high-rate filter in the treatment train.
(4) Upgrading an existing two-stage trickling filter. The following options are available for
upgrading an overloaded two-stage filter:
-- Construction of a roughing filter preceding the existing system.
-- Construction of a separate biological treatment system parallel to the existing facilities.
b. Upgrading to increase organic removal efficiency. The selection of an upgrading technique is based
on the ability of the existing facilities to handle increased hydraulic or organic loads. Modifications are
provided to meet higher effluent standards even though the existing facilities are not hydraulically or
organically overloaded. Table 8-1 contains approved alternatives for improving effluent quality under these
conditions and suggests anticipated improvements in performance for each alternative. In cases where unit
processes are added to existing facilities, it must be emphasized that the improvement in overall organic
removal will be a direct function of the biochemical oxygen demand removal achieved in the new downstream
unit process. Where unit processes precede existing units, however (such as with the use of a roughing filter),
the overall biochemical oxygen demand removal may not be increased in direct proportion to the amount
achieved in the "add-on" process. A detailed discussion on polishing lagoons, microscreens, filters, activated
carbon, clarifiers, and land treatment modifications appears in subsequent chapters. The applicability of each
alternative in individual upgrading cases should be evaluated in detail in a feasibility study.