What DAO 2021-19 Changed, and Why Our Designs Did Not
Most people assume environmental regulation only tightens. In the Philippines it loosened once, and a lot of sewage treatment plants quietly started passing without anyone touching them.
Under DAO 2016-08, signed in May 2016, ammonia was capped at 0.5 mg/L for Classes A, B, C, SA, SB and SC. That is a brutal number. It means full, stable nitrification with nothing going wrong on the day of sampling. Phosphate sat at 1 mg/L for the same classes.
In June 2021, DAO 2021-19 updated selected parameters — ammonia, phosphate, fecal coliform, boron, copper and sulfate — and several of the nutrient limits became less stringent.
| Parameter | Class | 2016-08 | 2021-19 |
|---|---|---|---|
| Ammonia (NH3-N) | A | 0.5 | 2 |
| B | 0.5 | 3 | |
| C | 0.5 | 4 | |
| D | 7.5 | 9 | |
| SB | 0.5 | 3 | |
| SC | 0.5 | 4 | |
| Phosphate | B | 1 | 1.5 |
| C | 1 | 4 | |
| SB | 1 | 2 | |
| SC | 1 | 4 |
It is worth being precise about what did not happen. Nitrate did not change; it is still 14 mg/L for Classes B and C. BOD and TSS did not change. And not every revision was relief: copper was redefined from dissolved copper to total copper, which is a stricter way of measuring the same element. The full set of current limits by class is on our compliance page.
One correction worth carrying, because we hear it often: Class SB did not become equivalent to Class C. SB allows 3 mg/L ammonia against C's 4, and 2 mg/L phosphate against C's 4. SB is the stricter of the two.
Clients who had been failing by a small margin started passing. They changed nothing. The number moved.
Our designs did not move with it
That was deliberate rather than stubborn.
A plant designed to produce exactly the legal limit passes only on days when nothing goes wrong, and something goes wrong regularly. Flow changes. Occupancy changes. Somebody empties bleach into a drain. A diffuser clogs. The operator resigns. A design with no margin converts every ordinary bad day into a failed certificate, and then into a site visit, and then into an argument about whose fault it is.
There is a second reason, and it is commercial rather than technical. The stricter design is the one that leaves reuse on the table later. Water treated down to a discharge minimum is water you can only discharge. Whether reuse actually pays on your site is arithmetic worth doing early, because retrofitting for it costs considerably more than designing for it.
We also do not charge extra for the margin. It is easier to carry a slightly larger tank in the original design than to return to a plant for two years trying to make a system that runs at its absolute limit somehow stay compliant.
Which standard applies to you is our work, not yours
None of the above matters until you know which class your receiving water is. That single fact decides which column of the table you are being judged against, and it is decided before any equipment is chosen.
We treat establishing it as part of our scope. Asking an owner to go and find out from DENR which class their outfall discharges to, and then designing to whatever they come back with, moves engineering risk onto the person least equipped to carry it. We determine it, we state it in the quotation, and where written confirmation is needed we prepare and file it at our cost.
If you are not certain which standard your plant is actually being held to, or you suspect the answer changed while nobody was looking, that is the first thing we would check. Book a free 15-minute consultation.
Figures above are taken from the text of DENR Administrative Order 2016-08 and DENR Administrative Order 2021-19. Your applicable limits depend on your receiving water classification and, for some industries, on parameters specific to your sector.