FABE 652
Ecosystems for waste treatment

Lectures #3 & 4 A case study of the Burlington, Vermont AEES
April 8 & 9, 2003

I. EXECUTIVE SUMMARY

Design concept:

Incorporates vascular plants, invertebrates, and fish into modified activated sludge extended aeration treatment process followed by clarification and filtration. Use both microbial community attached to plant roots, and suspended bacteria to affect nutrient removal in aerated complete mix reactors prior to clarifier. After clarifier higher inverts (snails, micro-crustacea, fish) are used to consume residual biosolids.

(Main differences with conventional waste water treatment plant are highlighted in blue.)

Summary characteristics

Treats 80,000 gal/day (300 m3/d) (150m3/d each line) raw domestic wastewater to advacnced tertiary standards. Addressed CBOD, TSS, TKN, NH3, NO3, TN, Fecal Coliform, (not P) (operation 1996-1999). 2-40,000 gal/day treatment lines with HRT of 2.9 days. 7800 ft² greenhouse. (This is 100’ x 78’ roughly 1/3 of soccer or football field.)

Order of components (Figure 2.1)

Anoxic reactor (9hr HRT)

Series of 5 aerobic reactors (9 hr HRT-each). Recycle from aerobic to anoxic reactors. Complete nitrification, denitrification, and CBOD removal.

Clarifier. Biosolids are either wasted or recycled to anoxic reactor. Complete TSS removal.

3 ecological fluidized beds (EFB) in series (HRT 6 hr)-recirculating submerged trickling filters. Air lift pumps provide circulation. Effluent discharged without disinfection.

Test train 5000 gal/day

Role of Plants:

Grow roots 2 ft. deep. VT tanks were 13 ft. deep—too deep. Plant roots should occupy minimum of 20-30% water column to provide significant treatment capacity. Better COD results with greater % of plant root volume.

Clarification

BOD (biological oxygen demand)-both carbonaceous OD and nitrogenous (and other chemical oxygenation) OD. Nitrogenous OD is NOT inhibited

CBOD (Carbonaceous biological oxygen demand)- Nitrogenous OD IS inhibited

COD (chemical oxygen demand)= BOD-CBOD

Operations and Maintenance:

15% less than conventional WWTP

II. DESIGN BASIS & TREATMENT PERFORMANCE ANALYSIS

Design standards Table ES.1

Aerated Reactors:

Volume of each = 57m3 (14,900 gal) HRT each is 9 hr (total of 5 is 45 hr)

Function: nutrient removal, use return activated sludge, first reactor was changed to anoxic to improve N removal.

Clarifier and solids holding tank (SHT):

Volume= 25m3 (6600 gal) HRT is 4 hr

Accumulated solids are pumped by air lift pump to SHT then to first reactor or removed.

Function: settle and remove biosolids

Ecological Fluidized Beds (EFB):

These are recirculating, downflow, vertical rock filters. Liquid volume 39m3 (10,300 gal) HRT in each EFB is 6 hr. first EFB 1.5" diameter lava rock, second and third EFB .75" diameter lava rock. Media cleaned by air-scour backflush. These EFBs use non-buoyant media (unlike other living machines). Recirculation rate is 3-5Q by airlift pumps. Second EFB is anoxic to promote denitrification. Methanol dosing was discontinued for 2^nd EFB.

Function: Nitrification/denitrification and final polishing of effluent.

CBOD (carbonaceous biochemical oxygen demand) Removal:

Effluent below 10 mg/L (w/out methanol) influent 230 mg/L (Table 2.5) (Figure 2.5)

Media bed clogging also a problem leading to higher CBOD effluent

COD (Chemical Oxygen Demand) Removal:

Effluent below 30 mg/L (w/out methanol) Figure 2.11 (similar to CBOD)

TSS (Total Suspended Solids) Removal:

Effluent below 5 mg/L (w/out methonal) influent 200 mg/L (Figure 2.15)

Nitrogen transformation and removal (explain N cycle):

NH3 Effluent 0.3 mg/L influent 25 mg/L

NO3 5 mg/L

TKN 1.5 mg/L 27 mg/L

TN 10 mg/L 31mg/L(84% mass removal)

TKN (total Kjeldahl) measures organic N and NH3 (reduced forms). Almost all organic N is converted to NH3. Through nitrification (aerobic) converted to nitrate. Nitrate converted to N gas via denitrification (anaerobic).

TKN removal is dependent upon nitrification in aerobic reactors (fig. 2.17). Very stable removal.

Nitrification similar to TKN very stable dependent on aerobic reactors (fig 2.22). insensitive to cold water inputs.

Denitrification—different phases due to changes in design (Table 2.14)(fig. 2.26)

Denitrification is biological process-bacteria use NO3 as electron acceptor to metabolize C. Must have C for bacteria respiration and cell growth. Must occur on anoxic (Dissolved oxygen DO <0.4 mg/L).

Phase 1: denitrify in second EFB using residual C as substrate. Failed because of insufficient carbon substrate in 2^nd EFB.

Phase 2: use methanol as C substrate in 2^nd EFB. Better results, but negative impacts: 2^nd and 3^rd EFB fouled due to biofilms growing on methanol substrate and added costs of methane addition.

Phases 3-5: Made first aerobic reactor anaerobic and returned sludge and nitrified effluent at 3 times influent flow rate from last aerobic reactor to new anaerobic reactor. This worked and produced effluent NO3 below 5 mg/L.

Total N—Figure 2.32

Phosphorus Removal:

Effluent-2.1mg/L influent 6mg/L (EPA requires <1mg/L)

P removal achieved by biosolids removal-cannot stimulate bacteria into luxury P uptake. Additional treatment needed to meet 1mg/L std. (Fig. 2.36)

Fecal Coliform Removal Performance:

Effluent: 1700 colony forming units Influent 8,500,000 CFU/100ml.

Mechanisms of removal are EFBs with filtration (1 order magnitude) clarifier, grazing by zooplankton (?).

With disinfection could meet standard for surface water discharge or reuse.

III. SYSTEM COMPONENT ANALYSIS

Aerated Reactors (AR):

The 5 AR remove CBOD, COD, TKN, NH3, NO3 to design standards (this accounts for the first AR being anaerobic). Total volume (all 5)=226 m3 HRT of 4 AR is 1.5 days

Use a standard wastewater equation to model CBOD, COD, NH3(1^st order CFSTR):

Fig. 3.1 model fit and removal of CBOD (stds. Met in 4^th AR)

Almost identical for COD-stds met in 4^th in AR

Addition of Recycled Activated Sludge (RAS) significantly improved nitrification performance. Nitrification neg. affected by clogged air diffusers. Nitrification complete by 4^th AR. (Figure 3.6)

Clarifiers:

Passive, center-feed, circular clarifiers used. Side wall depth 9 ft. Supernatant through a submerged perforated pipe to first EFB. 30 degree bottom cone collects biosolids with removal via an airlift pump to SHT. Problem-solids did not settle well in 30 degree cone. Led to gas production in sludge blanket and "floating" of sludge. Operation improved with the change to anaerobic reactor at start of system. Maybe because N gas was resuspending the sludge??

Ecological Fluidized Beds:

These are submerged, recirculating rock filters

The lava rocks did not "fluidize" during air-scour backflushing-(not well described) and is "simply a submerged, recirculating trickling filter."

Initially designed for nitrification and denitrification. After RAS cycle in AR main purpose is to remove TSS.

The RAS cycle in AR outperforms nit/denit in EFBs. They easily provided added polishing for TSS—redundant.

0.75" media occasionally clogged 1.5" media (lava rock) did not clog.

No apparent change in treatment from 5 ft. to 10 ft. medium ht.

Aeration:

Provided by two rotary blowers-165 ft3/min at 12 psi

Plants: (fig 3.13)

Plants grow on racks, identify suitable species, tanks too deep to optimize exposure to suspended roots.

AR 2-5 are planted, racks cover 60% of reactor surface, each rack baffled to prevent root damage from turbulence, water cycled down through each root zone with air-lift pumping (Figure 3.14)

Design Basis: to provide large surface area on root mass for attachment of biofilms. Unknown how this affects treatment—assumed to be positve.

Plant roots grow to depth of 2ft. and should occupy about 20-30% water column (only 9% here)

Table 3.4 best plants for these systems (tested in this study)

Plant Mgt.: Pest mgt. And harvesting.

Pest include aphids, spider mites, thrips, white flies-spray with water or remove affected part of plants, "natural" fungal insecticides. Prune dead, diseased, overgrown plant material. Species diversity limits pest-try to maintain diveristy.

Macrofauna:

Includes fish, snails, adult crustacea plankton (copepods & amphipods)

No systematic study only observations-

Fish only survive in EFBs-they die in clarifiers (EFBs ???)

Snails densely cover wetted surfaces of clarifier, EFBs, and plants in the EFBs. Snails also on plant roots in 4 and 5 AR. Snails graze on biofilms.

Large populations on copepods and amphipods live in rock medium of EFBs. Amphipods also in plant roots. Both known to graze on suspended bacteria and biofilms.

IV. TEST TRAIN STUDIES:

Planted vs. Unplanted aerobic reators (Figure 4.1):

Aquamat media used in unplanted AR

Planted AR better NH3 & NO3 (figure 4.4), TN (fig. 4.5)

No differences for CBOD and P removal

250 gal biosolids wasted without plants, only 20 gal with plants: possibly due to biosolids retained in roots and grazing by snails etc.

Plants covered 100% area of AR

EFB media:

Found Jeager rings and "scrubbies" to yield better results than lava rock—recommend Jeager rings for EFBs.

V. OTHER TOPICS

Costs conv. Treatment system =$82680

AEES system =$70625 (15% savings)