Bardenpho stages one sludge through alternating electron-acceptor environments so a single biomass removes carbon, nitrogen and — in the five-stage version — phosphorus. This covers both configurations, the zone chemistry underneath them, design and operating ranges, and the failure mode that specifically defeats five-stage plants.
This covers the Bardenpho process specifically — both the four-stage nitrogen configuration and the five-stage (modified) configuration that adds biological phosphorus removal. It assumes you know activated sludge fundamentals: SRT, MLSS, RAS, F/M. The quantitative content comes from the U.S. EPA Nutrient Control Design Manual, EPA/600/R-09/012, the authoritative U.S. public reference on biological nutrient removal. Where a number comes from elsewhere, it's labeled.
Bardenpho is a single-sludge activated sludge process that stages the biomass through alternating electron-acceptor environments so that one population of organisms accomplishes carbon oxidation, nitrification, denitrification and — in the five-stage version — luxury phosphorus uptake.
Everything that follows is a consequence of that idea. The zones exist because different metabolic pathways require different terminal electron acceptors, and the recycles exist because the products of one zone are the substrates of another.
| Process | Zones | Removes | Typical effluent TN |
|---|---|---|---|
| MLE (Modified Ludzack-Ettinger) | Anoxic → Aerobic, internal recycle | Nitrogen only | 5–8 mg/L, ~80% TN removal at 2–4Q recycle |
| 4-stage Bardenpho | Anoxic → Aerobic → Anoxic → Re-aeration | Nitrogen only | 3–5 mg/L |
| A/O (Pho-redox) | Anaerobic → Aerobic | Phosphorus only | n/a |
| A2/O | Anaerobic → Anoxic → Aerobic | N and P | Moderate |
| 5-stage Bardenpho | Anaerobic → Anoxic → Aerobic → Anoxic → Re-aeration | N and P | Lowest of the conventional configurations |
| UCT / Modified UCT | Recycles reconfigured to shield the anaerobic zone from nitrate | N and P | Comparable to 5-stage |
| Johannesburg (JHB) | Anoxic RAS treatment ahead of the anaerobic zone | N and P | Comparable |
Source: EPA Nutrient Control Design Manual (EPA/600/R-09/012), Sections 5.2.1.1 and 6.2. MLE performance figures and the 4-stage TN range of 3–5 mg/L are stated directly in that document.
EPA states it plainly: the first two stages of the four-stage Bardenpho are identical to MLE — an anoxic zone followed by an aeration zone with a nitrate-rich recycle back to the anoxic zone. The third and fourth stages are what Bardenpho adds: a secondary anoxic zone that denitrifies the portion of flow the internal recycle never captured, and a re-aeration zone. That's the entire structural difference — and it's why a plant already running MLE is the natural candidate for a Bardenpho conversion. The front half already exists.
Bardenpho works by giving the same biomass three different electron-acceptor conditions in sequence. Getting the terminology exactly right matters, because the distinction between anoxic and anaerobic is the distinction between nitrogen removal and phosphorus removal.
| Zone | Free O₂ | Nitrate/nitrite | Electron acceptor | What happens |
|---|---|---|---|---|
| Aerobic | Present | Produced here | Dissolved oxygen | Carbon oxidation, nitrification, P uptake |
| Anoxic | Absent (below ~0.2–0.5 mg/L) | Present | Nitrate or nitrite | Denitrification |
| Anaerobic | Absent | Absent — the critical requirement | Neither | VFA uptake and P release by PAOs |
This is the single most consequential definition in the process. An "anaerobic" zone receiving nitrate is functionally an anoxic zone, and the phosphorus removal it was built to accomplish will not happen. The mechanism: denitrifiers preferentially consume the readily biodegradable COD that phosphorus accumulating organisms need. EPA notes the competition directly — denitrification of nitrate recycled to anaerobic zones reduces the RBCOD available for biological P removal. This is why UCT, Modified UCT, Johannesburg and the five-stage Bardenpho all exist as separate configurations. Each is a different engineering answer to the same problem: shielding the anaerobic zone from recycled nitrate.
| Per gram of NH₃-N oxidized to nitrate | Value |
|---|---|
| Oxygen consumed | 4.57 g |
| New cells formed | 0.16 g |
| Alkalinity destroyed | 7.14 g as CaCO₃ |
| Inorganic carbon used in cell formation | 0.08 g |
| Nitrification condition | Requirement or effect |
|---|---|
| Organisms | AOB — chiefly Nitrosomonas, also Nitrosococcus, Nitrosospira. NOB — chiefly Nitrobacter, also Nitrospina, Nitrococcus, Nitrospira. |
| Growth rate | Maximum specific growth rate is 10–20× lower than heterotrophs oxidizing carbonaceous BOD. This is why nitrification sets the SRT. |
| Design SRT | 10–20 days at 10 °C; 4–7 days at 20 °C, with sufficient DO and adequate pH |
| Dissolved oxygen | Rates decline below 3–4 mg/L, significantly below 2 mg/L. At 0.50 mg/L the rate is ~60% of the rate at 2.0 mg/L. |
| pH | Operates well between 6.8 and 8.0. Near pH 6.0 the rate may be only ~20% of the rate at pH 7.0. |
| Temperature | Between ~10 and 25 °C the rate approximately doubles for every 8–10 °C increase. |
Source: EPA Nutrient Control Design Manual, Sections 5.1.1 and 5.3, citing Randall et al. (1992), Tchobanoglous et al. (2003), WEF and ASCE (2006).
| Parameter | Value |
|---|---|
| Alkalinity recovered | 3.57 mg/L as CaCO₃ per mg/L NO₃-N consumed — roughly half what nitrification destroys |
| Carbon requirement | Rule of thumb: ~4 g influent BOD per g NO₃-N removed |
| Biomass yield | ~0.4 g VSS per g COD consumed |
| DO inhibition threshold | Inhibited above ~0.2–0.5 mg/L DO; reported range across sources 0.10–0.50 mg/L |
| Organisms | Facultative heterotrophs, widely distributed — Pseudomonas, Bacillus, Alcaligenes, Achromobacter, Flavobacterium and many others |
Nitrification destroys 7.14 g of alkalinity per gram of ammonia-N oxidized. Denitrification returns 3.57 g per gram of nitrate-N reduced — EPA notes alkalinity is partially replenished, up to 62.5 percent. A plant that nitrifies without denitrifying frequently buys that alkalinity back as caustic, lime or soda ash. A Bardenpho plant recovers most of it in-process. Run the alkalinity balance before assuming you need chemical addition — on many plants the second anoxic zone is what keeps pH in range.
| # | Zone | Receives | Function |
|---|---|---|---|
| 1 | Primary anoxic | Influent, RAS, internal mixed liquor recycle | Denitrification using influent BOD — the cheapest carbon you will ever have |
| 2 | Aerobic | Anoxic zone effluent | Nitrification and removal of remaining BOD. Internal recycle drawn from the end of this zone. |
| 3 | Secondary anoxic | Aerobic zone effluent | Denitrifies the fraction of flow the internal recycle didn't capture. Supplemental carbon may be dosed here. |
| 4 | Re-aeration | Secondary anoxic effluent | Strips nitrogen gas and raises DO before clarification |
The re-aeration zone is often misunderstood as a polishing step. It isn't primarily about treatment — EPA describes its function as stripping nitrogen gas and increasing DO before clarification.
Nitrogen gas bubbles generated in the secondary anoxic zone will attach to floc and float it. Denitrification in the clarifier blanket is a classic cause of rising sludge, and it doesn't look like a bulking problem — the sludge settles, then comes back up in clumps. Sending low-DO mixed liquor to a clarifier also risks the blanket going anaerobic, which in a five-stage plant causes secondary phosphorus release. If you're seeing intermittent solids carryover with good SVI, look at re-aeration detention time and DO before you look at the clarifier.
The primary anoxic zone can only denitrify the nitrate delivered to it. With an internal recycle ratio of R, maximum theoretical nitrate removal in that zone is R/(R+1+RAS ratio) of what was produced. Increasing R gives diminishing returns and carries penalties.
| Consequence of raising internal recycle | Effect |
|---|---|
| More nitrate delivered to the primary anoxic zone | Higher TN removal — the intended effect |
| More dissolved oxygen carried over with it | Consumes carbon that would otherwise drive denitrification; can push the anoxic zone above its DO threshold |
| Higher pumping energy | Internal recycle is typically the largest recycle stream in the plant |
| Shorter effective anoxic contact time | Higher flow through a fixed volume |
EPA cites internal nitrate recycle ratios of 2–4Q for MLE, achieving ~80% TN removal and 5–8 mg/L effluent TN. The four-stage Bardenpho reaches 3–5 mg/L by adding the second anoxic zone rather than raising recycle further — which is the central design insight of the process.
By the time mixed liquor reaches the secondary anoxic zone, readily biodegradable carbon is largely gone. EPA notes that when denitrification occurs after secondary treatment there is little BOD remaining, so a supplemental carbon source is often needed — and that plants meeting very low TN limits typically use a secondary anoxic zone with supplemental carbon addition.
| Carbon source | Notes from EPA |
|---|---|
| Methanol | Most common external source because of low cost. Drawbacks: highly flammable and implicated in storage tank fires and explosions; not the most efficient source for most configurations; widely fluctuating costs; regional availability problems; reported low growth rates at cold temperatures. |
| Ethanol, acetic acid, glycerol | Alternatives in active use |
| Corn syrup, molasses, glucose | Also used; selection depends on facility needs |
| Industrial waste products | Site-specific opportunity |
| Internal sources | Fermented wastewater or fermented sludge — avoids purchased chemical entirely |
The obvious cost is chemical. The less obvious one is that unconsumed carbon passes into the re-aeration zone and the clarifier, appearing as effluent BOD and consuming oxygen you paid for. Methanol-fed systems in particular need dosing control tied to actual nitrate load, not flow alone. One Connecticut system identified control of methanol feed based on influent COD as a key design issue.
The modified Bardenpho places an anaerobic zone at the head of the four-stage train. That single addition converts a nitrogen removal process into a combined nitrogen and phosphorus removal process, because it creates the anaerobic contact that enhanced biological phosphorus removal requires.
| # | Zone | Function |
|---|---|---|
| 1 | Anaerobic | PAOs take up volatile fatty acids, store them as PHA, and release orthophosphate. No oxygen, no nitrate. |
| 2 | Primary anoxic | Denitrification on influent carbon and internal recycle nitrate |
| 3 | Aerobic | Nitrification, BOD removal, and luxury phosphorus uptake by PAOs |
| 4 | Secondary anoxic | Polishing denitrification |
| 5 | Re-aeration | Nitrogen gas stripping and DO restoration before clarification |
Enhanced biological phosphorus removal depends on selecting for phosphate accumulating organisms by cycling them between anaerobic and aerobic conditions. The sequence is what makes it work:
Source: EPA Nutrient Control Design Manual, Section 6.1 and Figure 6-1, "Theory of biological phosphorus removal in activated sludge."
1. Rising soluble phosphorus in the anaerobic zone is normal and required. It's evidence the process is working, not evidence of failure.
2. Phosphorus is removed by wasting sludge. If you stop wasting to build MLSS, you stop removing phosphorus — and the P you accumulated stays in the system.
3. Anything that holds P-rich sludge under anaerobic conditions without a subsequent aerobic zone will release that phosphorus back into the liquid stream. That includes the clarifier blanket, gravity thickeners, and anaerobic digesters — which is why sidestream P return is a chronic issue at EBPR plants.
Both PAOs and denitrifiers want the same readily biodegradable COD. If nitrate reaches the anaerobic zone, denitrifiers consume the VFAs there and the PAOs are starved of the substrate they need to release phosphorus and store PHA. This is the central design tension in every combined N and P removal process.
| Design response | How it addresses the competition |
|---|---|
| 5-stage Bardenpho | Places the anaerobic zone first, ahead of the internal recycle return point, so recycled nitrate enters the anoxic zone instead. RAS nitrate remains a residual concern. |
| UCT / Modified UCT | Recycles from the anoxic zone to the anaerobic zone rather than returning RAS directly, so the anaerobic zone receives a low-nitrate stream |
| Johannesburg (JHB) | Passes RAS through a dedicated anoxic zone to strip nitrate before it reaches the anaerobic zone |
| Primary sludge fermentation | Increases the VFA supply so there's enough carbon for both populations |
EBPR is not always achievable on a given wastewater. The controlling variable is how much readily biodegradable carbon is available per unit of phosphorus. EPA treats COD:P ratio as the first operational and design consideration for biological phosphorus removal — ahead of retention time and temperature.
A dilute or highly infiltrated influent, a long collection system that has already fermented, or heavy primary clarification can all leave insufficient rbCOD at the anaerobic zone. Remedies in order of typical cost: bypass some primary treatment to preserve influent carbon; ferment primary sludge to generate VFAs on site; add external carbon; or supplement with chemical phosphorus precipitation. EPA explicitly identifies hybrid chemical/biological processes as an established configuration — running EBPR for the bulk of the load with metal salt trim for the last fraction is a legitimate design, not an admission of failure.
Figure 6-8 of the EPA Nutrient Control Design Manual is titled "Example of Secondary Release in Second Anoxic Zone." In the later edition it appears as Figure 5-4 under the same title. The fact that EPA illustrated this specific failure in this specific zone tells you how commonly it defeats five-stage plants.
The five-stage Bardenpho asks the second anoxic zone to finish denitrification. But once nitrate is exhausted in that zone, the environment is no longer anoxic — it is anaerobic. Phosphorus-rich PAOs sitting in an anaerobic environment with no subsequent substantial aerobic zone will do what PAOs do: release phosphorus.
That released phosphorus then passes through a short re-aeration zone that may not provide enough contact time for meaningful re-uptake, and leaves in the effluent. The plant meets its nitrogen limit and misses its phosphorus limit — and the cause is invisible unless you are profiling soluble P across the zones.
| Contributing condition | Corrective direction |
|---|---|
| Second anoxic zone oversized relative to actual nitrate load | Reduce effective volume or bypass part of it. Zone volume that outlasts the nitrate is working against you. |
| Nitrate exhausted early in the zone | Reduce or stop supplemental carbon dosing; verify dosing is nitrate-load-paced |
| Insufficient re-aeration contact time | Increase re-aeration detention or DO to allow P re-uptake before clarification |
| Deep clarifier blanket | Increase RAS rate to reduce sludge detention time in the blanket |
| Long RAS residence | Same — minimize time P-rich sludge spends without an electron acceptor |
| Sidestream returns | Thickener and digester supernatant carry released P back to the head of the plant. Quantify the load before blaming the mainstream process. |
Corrective directions are process reasoning from the EPA-documented mechanism, not a published EPA remediation table. Verify against your own profile data and design conditions.
The values below are typical ranges compiled from EPA guidance and peer-reviewed literature. They are orientation, not a design basis. Actual zone sizing depends on influent characterization, temperature, target effluent limits, and modeling — EPA devotes an entire chapter to mathematical modeling for exactly this reason. Where sources conflict, both figures are shown.
| Zone | Typical HRT | Note |
|---|---|---|
| Anaerobic | 0.5–1 h (one review); 1–2 h (pilot optimization) | The carbon competition makes this zone's contact time critical. Undersizing starves the PAOs. |
| Primary anoxic | 1–2 h | Sized to the nitrate load delivered by internal recycle |
| Aerobic | Set by nitrification SRT requirement | This is the zone that sets total system size |
| Secondary anoxic | 2–4 h | Note the secondary release risk — larger is not better here |
| Re-aeration | 0.5–1 h | Enough to strip N₂ and restore DO |
| Total system | ~15–16 h in reported pilot work | Varies widely with loading and temperature |
HRT ranges from a peer-reviewed integrated UASB-Modified Bardenpho study (2024); pilot optimization figures from Bahrami et al., hybrid five-stage Bardenpho-MBBR study, Journal of Environmental Chemical Engineering (2018). Both are research studies rather than design standards.
| Parameter | Value | Source |
|---|---|---|
| Nitrification design SRT at 10 °C | 10–20 days | EPA, citing Randall et al. (1992) |
| Nitrification design SRT at 20 °C | 4–7 days | EPA, citing Randall et al. (1992) |
| Typical Bardenpho system SRT | 10–25 days | Peer-reviewed literature |
Nitrification requires a long SRT — nitrifiers grow 10–20× slower than heterotrophs, and washing them out is the fastest way to lose ammonia compliance. EBPR generally favors a shorter SRT, because longer sludge age means more endogenous decay and a lower fraction of the biomass as active, P-storing PAOs. You cannot optimize both independently in a single-sludge system. That is the fundamental compromise of combined BNR, and it's why cold-weather operation is where five-stage plants are most often tested — the SRT you need for winter nitrification is not the SRT that maximizes P removal.
| Stream | Typical ratio | Function |
|---|---|---|
| Internal (nitrate) recycle | 2–4Q per EPA for MLE; 200% (2Q) reported optimal in pilot work | Delivers nitrate from the aerobic zone to the primary anoxic zone |
| RAS | Plant-specific, commonly 0.5–1.0Q | Returns biomass; in a 5-stage plant also returns whatever nitrate survived |
| WAS | Set by SRT | The phosphorus removal mechanism |
For EBPR, what matters is not just detention time but the fraction of total biomass exposed to anaerobic conditions. Reported values vary substantially — one comparison cites a conventional A2/O process at roughly 7.5% anaerobic mass fraction against roughly 22% for a RAS fermentation configuration, and characterizes a 7% anaerobic zone as too small.
Source: Dold, P., "Achieving Enhanced Biological P Removal: Have We Forgotten How to Design a BioP Plant?" (2019), citing Gu et al. (2019) WRF study. Industry technical paper.
| Symptom | Likely causes | Checks |
|---|---|---|
| Ammonia breakthrough | Insufficient aerobic SRT; DO too low; pH depressed; temperature drop; toxic inhibition | Verify aerobic SRT against temperature. DO profile — rates decline below 3–4 mg/L, sharply below 2. Check pH against 6.8–8.0. Review industrial discharges. |
| Effluent nitrate high, ammonia fine | Denitrification limited — insufficient internal recycle, insufficient carbon, or DO carryover into the anoxic zone | Check internal recycle ratio. Measure DO at the anoxic zone inlet — above 0.2–0.5 mg/L inhibits denitrification. Verify the ~4 g BOD per g NO₃-N carbon availability. |
| Nitrite accumulation | NOB inhibited more than AOB, often by low DO | EPA notes that below 0.5 mg/L DO the effect is greater for Nitrobacter than Nitrosomonas. Also watch chlorine demand — 1 g NO₂-N consumes 5 g chlorine. |
| pH falling through the plant | Alkalinity destroyed by nitrification exceeding what denitrification returns | Run the balance: 7.14 g destroyed per g NH₃-N oxidized, 3.57 g returned per g NO₃-N reduced. Improving denitrification may fix pH without chemical addition. |
| Rising sludge, good SVI | Denitrification in the blanket generating N₂ bubbles | Check re-aeration performance and clarifier nitrate. Increase RAS to reduce blanket detention. |
| Symptom | Likely causes | Checks |
|---|---|---|
| Effluent P high, N compliance fine | Nitrate intrusion into the anaerobic zone; insufficient rbCOD; secondary release | Profile soluble P and nitrate across all zones. This single test distinguishes all three causes. |
| No P release in the anaerobic zone | Nitrate or DO present; insufficient VFA | Measure nitrate and DO at the anaerobic zone. If both near zero, the problem is carbon. |
| Good release, poor uptake | Insufficient aerobic contact or DO; PAO population stressed | Check aerobic DO and detention. Verify wasting rate. |
| P climbing without process change | Sidestream return load from thickening or digestion | Sample thickener and digester supernatant. Released P returns to the head of the plant. |
| P removal degrades at long SRT | Reduced active PAO fraction; possible GAO competition | Review SRT against the nitrification requirement — this is the compromise, not a fault. |
| Seasonal P performance swing | Temperature effects on EBPR kinetics | EPA lists temperature among the primary EBPR design considerations. |
A full soluble orthophosphate and nitrate profile across every zone, on the same day, at steady loading. Anaerobic P release, aerobic P uptake, nitrate consumption through the anoxic zones, and any P rebound after the aerobic zone. That single dataset distinguishes nitrate intrusion, carbon limitation, and secondary release — three problems with completely different fixes and identical effluent symptoms.
| Favors Bardenpho | Favors an alternative |
|---|---|
| Low effluent TN required — four-stage reaches 3–5 mg/L against MLE's 5–8 | TN limit is moderate and MLE will meet it at lower cost and complexity |
| Combined low TN and TP required (five-stage) | Phosphorus only — A/O is simpler |
| Tankage already available, or an existing MLE train to extend | Severely space-constrained site — each zone requires dedicated volume |
| Carbon-rich influent supporting EBPR | Carbon-poor influent — consider chemical P or fermentation first |
| Operating staff capable of BNR process control | EPA notes BNR requires advanced knowledge and much greater process control than BOD and TSS removal alone |
| Desire to recover alkalinity in-process | — |
EPA identifies the zones themselves as the critical component and cost. Each of the five zones requires dedicated tank space, and a plant without existing tankage is buying concrete before it buys anything else. EPA also flags two systemic considerations for advanced nutrient removal: increased carbon footprint from higher energy use, and susceptibility to wet weather, cold weather and inhibitory substances.
| Configuration | Reported result | Source type |
|---|---|---|
| 4-stage Bardenpho | Effluent TN 3–5 mg/L | EPA design manual |
| MLE (comparison) | Effluent TN 5–8 mg/L, ~80% TN removal | EPA design manual |
| 5-stage at a U.S. WRF | TP to 0.2 mg/L; TN from 30 mg/L down to 8.0 mg/L | Trade press case coverage |
| 5-stage hybrid with MBBR (pilot) | TN ~92.5%, TP ~94.7%, COD ~98.2%, NH₄-N ~96.5% removal | Peer-reviewed study |
| Integrated UASB + modified Bardenpho (research) | Effluent NH₄-N 0.8–1.2 mg/L, TN 5.1–7.9 mg/L | Peer-reviewed study |
Research figures are from controlled studies on specific wastewaters and should not be read as expected full-scale performance. EPA design manual ranges are the appropriate planning basis.
| Parameter | Value |
|---|---|
| O₂ per g NH₃-N nitrified | 4.57 g |
| Alkalinity destroyed per g NH₃-N nitrified | 7.14 g as CaCO₃ |
| Cells formed per g NH₃-N nitrified | 0.16 g |
| Alkalinity recovered per g NO₃-N denitrified | 3.57 g as CaCO₃ |
| Alkalinity replenishment fraction | up to 62.5% |
| BOD required per g NO₃-N removed | ~4 g |
| Biomass yield, denitrification | ~0.4 g VSS per g COD |
| Nitrifier growth rate vs heterotrophs | 10–20× slower |
| Nitrification SRT at 10 °C | 10–20 days |
| Nitrification SRT at 20 °C | 4–7 days |
| Nitrification DO | declines below 3–4 mg/L |
| Nitrification rate at 0.5 mg/L DO | ~60% of rate at 2.0 mg/L |
| Nitrification pH range | 6.8–8.0 |
| Nitrification rate at pH 6.0 | ~20% of rate at pH 7.0 |
| Nitrification temperature effect | rate ~doubles per 8–10 °C rise |
| Denitrification DO inhibition | above ~0.2–0.5 mg/L |
| Internal recycle ratio | 2–4Q (EPA, MLE basis) |
| 4-stage Bardenpho effluent TN | 3–5 mg/L |
| MLE effluent TN | 5–8 mg/L, ~80% removal |
| Typical Bardenpho SRT | 10–25 days |
| NO₂-N chlorine demand | 1 g NO₂-N consumes 5 g Cl₂ |
The primary source throughout is the U.S. EPA Nutrient Control Design Manual: State of Technology Review Report, EPA/600/R-09/012 (January 2009), prepared by The Cadmus Group with senior advisors Clifford Randall (Virginia Tech), James Barnard (Black & Veatch), David Stensel (University of Washington) and Jeanette Brown (Stamford WPCA). It's free, and anyone designing or modifying a BNR process should read it directly.
HRT ranges come from research studies, including one on synthetic vinasse rather than municipal wastewater — orientation only. Troubleshooting corrective actions are process reasoning from EPA-documented mechanisms, not a published EPA remediation table. Anaerobic mass fraction figures come from an industry paper citing a WRF study, not from EPA guidance. Performance figures from research and single facilities are not design guarantees — use EPA ranges for planning. Zone sizing, recycle ratios and SRT selection require site-specific influent characterization and, per EPA's own guidance, generally warrant process modeling. Effluent limits are set by your NPDES permit; nothing here is a compliance determination.
Activated Sludge: Bugs, Basins & Beyond includes a full subtopic on nutrient removal — nitrification and denitrification, washout SRT, alkalinity demand, IMLR, and biological phosphorus removal — inside 150 questions covering the whole activated sludge process, each with a detailed explanation.
Get the Activated Sludge Guide — $17Instant PDF download · 150 questions · Class I & II tagged