ESSI Engineering — Engineering Together
Water Treatment Engineering — CA · OR · WA · NC

Engineering
Together

At ESSI we believe in engineering a better future together — delivering world-class water treatment expertise with the personal service your project deserves.

50+
Projects Completed
15
Years Experience
4
States Licensed
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Water Treatment Design
Pilot Testing
Water System Planning
Operations Optimization
Water Reuse
ESSI Certification

World-Class Expertise.
Personal Service.

ESSI Consulting was established to bring drinking water systems world-class expertise on water quality and treatment — combining the depth found in large engineering firms with the responsiveness only a small firm can deliver.

Founded in 2022 and headquartered in Escondido, California, ESSI is licensed in California, Oregon, Washington, and North Carolina. Our team has designed or assisted in designing nearly 30 groundwater treatment plants across the West Coast, and has experience on more than 50 groundwater treatment projects.

Iron & Manganese Removal
Arsenic Removal
PFAS/PFOS
Chromium Removal
Nitrate Removal
Biological Treatment
Water Reuse
Corrosion Control
Pilot Testing
"Aaron Gress provided complex technical solutions in managing the only second pass agricultural drainage treatment facility in the United States. I would consider Aaron a great asset to any team looking for a dedicated individual who seeks creative and practical solutions to water treatment challenges."
Miguel Arias-Paic
Technical Practice Lead, United States Bureau of Reclamation
"Aaron Gress's meticulous approach to pilot testing has significantly advanced our water treatment processes, ensuring both efficiency and environmental compliance."
Lee Odell
Author, AWWA Treatment Technologies for Groundwater

Full-Spectrum Water Treatment Services

From preliminary planning through construction support and operations optimization — ESSI delivers at every stage of your project lifecycle.

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Pilot Testing
Full line of pilot testing for iron & manganese, arsenic, chromium, PFAS/PFOS, nitrate, ammonia, and more. Optimize your design before you build.
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Treatment Plant Design
Design of complete groundwater treatment plants — chemical feed systems, filtration, backwash handling, controls, and construction documents.
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Water System Planning
Water system plans, preliminary design reports, water supply analyses, rate studies, and bond financing reports tailored to your utility's needs.
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Operations Optimization
On-site evaluations, filter assessments, media sieve analysis, backwash optimization, process troubleshooting, and water quality audits.
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Water Reuse
Advanced treatment for reclaimed water — membrane filtration, RO, AOP, biological filtration, and electrochemical treatment solutions.
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ESSI Certification
The first engineering firm to offer an infrastructure certification program. ESSICERT.ORG recognizes projects delivering environmental and social equity.

50+ Projects Across
the West Coast

Request Full Project List →
28
Rockwood Water PUD — Gresham, OR
28 MGD Iron & Manganese Water Treatment Plant
One of the world's largest high-rate manganese removal plants · 136,000 residents served
Madera County, CA
Arsenic WTP — Coarsegold Elementary
Pilot testing & full design for arsenic, iron & manganese removal
City of Escondido, CA
65 MGD Treatment Optimization Study
PAC improvements & chemical dosing optimization for Lake Hodges supply
City of Newberg, OR
Iron & Manganese WTP — Pilot Study & Design
2023 · Planning, pilot testing, design
UIUC Tribe — Canyonville, OR
500 GPM Hypolimnetic Oxygenation & Ozone System
2022 · Full pilot to design
Licensed in:
California
Oregon
Washington
North Carolina
HQ: Escondido, CA · Also licensed in NC · Projects also completed in GA, UT, ID, CO, ND, HI, VA, AK, ON

Aaron Gress, PE, MSCE

Civil and environmental engineer with nearly 15 years of experience in water treatment planning, design, and construction. Author of several technical journal publications and a committee member of the AWWA Water Treatment Group.

15
Years of engineering experience with drinking water treatment across the West Coast
50+
Groundwater treatment projects completed from pilot testing through operations
20+
Water treatment plants designed — from 5 GPM wellhead systems to 85 MGD expansions
CA PE
OR PE
WA PE
NC PE
AWWA
WaterReuse
ASCE

Mr. Gress has managed projects with multiple subcontracts across municipal and commercial water treatment — from preliminary design through construction support. His expertise spans arsenic, iron & manganese, PFAS, chromium, nitrate, and biological treatment systems.

His approach emphasizes simple, cost-effective designs that engage operators and owners throughout the process — so teams know exactly what to expect when the plant is complete.

Also Founding ESSI Certification
ESSI Engineering is the first engineering firm to develop and offer an infrastructure certification program — recognizing projects that deliver environmental sustainability and social equity in underserved communities.
Learn about ESSICERT.ORG →

Let's Build Something Together

We respond promptly to all inquiries. Whether you have a specific project in mind or want to learn more about our services, we'd love to hear from you.

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✉️
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Web
essieng.com
📍
Location
Escondido, CA (Headquarters)
Also licensed in NC · CA · OR · WA

Tell Us About Your Project

We'll respond within one business day.