Purple Peril

Non-fiction · Public health

Purple Peril

How a Multi-Billion-Dollar Water Reuse Industry Threatens Public Health

by Mike Borrello

“Do not drink.” But what about breathing the aerosols? Recycled water meets the rules when it leaves the treatment plant. In the purple pipes beyond, biofilms can harbor Legionella—and sprinklers turn that water into mist in the places where people live, work and play.

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Paperback coming October 5

Book cover of Purple Peril: How a Multi-Billion-Dollar Water Reuse Industry Threatens Public Health, by Mike Borrello. The cover photo shows a recycled-water irrigation riser with a purple warning label standing among shrubs on a landscaped hillside.

About the book

“Do not drink.” But what about breathing the aerosols?

Across California and far beyond, cities pump treated wastewater through miles of purple pipe to irrigate parks, schoolyards, golf courses, and neighborhood landscapes. It’s sold to the public as a sustainability success, and leaving the plant it meets the rules. What happens beyond that point is barely watched. In the pipes between the plant and the spray heads, biofilm can harbor Legionella and other opportunistic pathogens, and emerging contaminants can pass through the plant largely undetected. Sprinklers then release whatever the water carries as a fine mist into the air people breathe.

Purple Peril is an investigative look at that gap. Drawing on peer-reviewed science, public-health data, cross-connection case histories, cost and investment records, and the author's own experience in a court petition against his city, it shows why "no evidence of harm" is not the same as evidence of safety. It does not argue against water reuse. It argues that a system which never measures and controls what reaches a lung cannot be called safe. It closes with practical fixes: monitoring where exposure happens, low- or no-aerosol irrigation where people gather, and honest public numbers.

This book is for utility managers, engineers, regulators, and public-health officials. It informs decision makers: city councils, water boards, school boards, parks directors, and HOA managers who approve and schedule the spray. It also equips the parents, teachers, groundskeepers, and residents who live downwind of wastewater aerosols with the knowledge to ask the right questions. If your decisions put recycled water into the air, or you live downwind of those decisions, this book is for you.

The problem

The rules stop at the plant. The exposure doesn’t.

Municipal water reuse has a blind spot. California’s Title 22 does not impose requirements to police the recycled-water distribution system—the “purple pipe” network that carries treated wastewater from the plant to the sprinkler head.

System diagram of a recycled-water system. Left: the treatment plant gate, labeled the Title 22 compliance point, where secondary effluent passes through sand or membrane filters, a chlorine contact basin and a UV reactor to become disinfected tertiary effluent; turbidity, flow, UV dose and chlorine residual are measured there, and a coliform grab sample has an approximately 24-hour delay and is not a live loop. Below it: Observability ends here. Right: a purple recycled-water pipe network, labeled typical 77 to 180-plus miles, Carlsbad about 97 miles, runs from a pump station past parks, a playground and a golf course, with a long-detention reservoir, a looped main and a dead leg with stagnation. Arrows mark residual decay, wall demand and biofilm, temperature rise, stagnation, pressure transients and sloughing, and open reservoirs. A box reads not installed: residual, coliform, Legionella. An exposure-pathways panel shows sprinkler aerosol cones, inhalable droplets, ponding, wet soil and open reservoirs reaching people near a playground.
Figure from Purple Peril From the plant gate to the spray head. The treatment plant gate is the Title 22 compliance point, and the diagram marks it plainly: “Observability ends here.” Beyond it, the recycled-water network (typically 77–180+ miles of pipe; Carlsbad ~97 mi.) carries water through dead legs and long-detention reservoirs, where residual decay, biofilm, temperature rise and stagnation take hold. Residual, coliform and Legionella monitoring in the network are marked “not installed.” Exposure happens through aerosol cones, inhalable droplets and ponding at parks and playgrounds. Select the figure to view it full size.

  1. Compliant at the plant

    Recycled water meets the rules as it leaves treatment. What happens to it in the miles of pipe beyond that point is barely watched.

  2. Biofilm in the pipes

    Biofilms can form on the walls of the distribution system and harbor pathogens such as Legionella.

  3. Sprayed into the air

    Irrigation releases that water as aerosols in parks, schoolyards and neighborhoods—air that people in urban communities breathe.

  4. Practical fixes

    The book closes with remedies: monitoring where exposure happens, low- or no-aerosol irrigation where people gather, and honest public numbers.

Illustrated cross-section titled 'Pipe-wall biofilm as a microscopic ecosystem.' Above a PVC or cement-lined iron pipe wall, a thick biofilm layer contains Pseudomonas and other heterotrophic bacteria, nitrifiers, water channels carrying nutrients in and waste out, a Vermamoeba or Acanthamoeba amoeba with Legionella replicating inside it, a nematode, fungal hyphae and a ciliate grazer. An oxygen gradient runs from an oxic channel to an anoxic base. At the top, cells slough off and disperse into the bulk flow of the pipe. A note reads: cells about 15 percent of volume; the rest is hydrated EPS and channels.
Figure from Purple Peril Pipe-wall biofilm as a microscopic ecosystem. Nutrients from the bulk water feed a layered community on the pipe wall. Legionella replicates inside amoebae living in the biofilm, and cells slough off and disperse into the stream that flows on toward the spray heads. Select the figure to view it full size.

The trend

Monthly case reports of Legionnaires’ disease, the illness caused by Legionella, climbed steeply after the early 2000s.

Bar chart of Legionnaires' disease monthly case reports from January 1982 to 2026, with cases on the vertical axis from 0 to 1,400. Monthly counts stay mostly below about 200 through the early 2000s, then rise sharply after about 2003, with frequent months above 400 and the highest peaks, above 1,200, around 2017. After 2017 the counts drop, then range up to about 500 in the most recent years.
Figure from Purple Peril Legionnaires’ disease monthly case reports, January 1982 to 2026. Monthly reports stayed mostly below about 200 through the early 2000s, then rose sharply after about 2003, peaking above 1,200 around 2017. Counts have fallen since that peak but remain higher than in the 1980s and 1990s. Data: compiled by the author from CDC online sources, 1982–2026. Chart created in LibreOffice. Select the chart to view it full size.

Why I wrote this

From city hall to the courthouse to the page

I recognized this gap through personal experience and research, and I decided something had to be said.

I started at home. I tried to engage my own city through council meetings and hearing protests. When the city failed to address the problem, I filed a petition for writ of mandamus on my own behalf, pro se, asking the city to show cause or cease and desist. The city prevailed on a demurrer, and the case was knocked out on process—the merits were never heard.

I could appeal, or I could walk away. Then I realized there was another way: write this book, and argue on a global stage the merits I tried to bring before the Superior Court.

My hope is that it gains enough traction that something is done to improve the health of urban communities.

That means one of two things: either solving the distribution problem—which I believe is economically intractable—or redirecting the billions sunk into purple pipe infrastructure toward improving the resiliency of our potable water systems.

— Mike Borrello

About the author

Mike Borrello

Mike Borrello is a retired engineer and scientist who spent most of his career in breathing life support. His work spanned undersea sport and commercial diving systems, closed-loop rebreathers for safety and rescue, and ventilators used in the ICU to maintain ventilation and oxygenation of the lung.

A career spent engineering what people breathe brings a particular question to recycled water: not only what is in it, but what reaches a lung.

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Coming October 5, 2026

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