Acerta Pioneers Real-Time Payroll

"The payroll engine is the beating heart of any payroll service provider," says Johan Guelluy, CIO at Acerta. We saw an opportunity to do more than just modernise ours: to build something in co-creation that could become the standard for the entire sector."

Acerta is one of Belgium's leading HR service providers, supporting employers, HR teams, and the self-employed across every stage of their professional journey. With an immense volume of payroll calculations processed every month, the margin for error is zero.

That reality is precisely what set an ambitious transformation in motion. In January 2020, Acerta's leadership decided to build an entirely new payroll engine. Guelluy, who had joined as CIO just weeks earlier, found himself in the driver’s seat of a technically and operationally challenging project.

A Moving Target

Payroll is a moving target and legislation changes every month. “You have to build a system that is always right, and that can absorb change constantly", says Guelluy. Belgian payroll law is notoriously complex. Sector-specific rules, tax obligations, and collective labor agreements are each capable of triggering a cascade of recalculations. Every legislative update must be absorbed immediately, without disruption.

Payroll is a moving target. Legislation changes every month.

On top of that, payroll processing has traditionally been a month-end sprint, a massive compression of workload into a narrow window. "In our solution, with a real-time engine, based on event-driven architecture, processing is spread across the entire month," Guelluy says. "The workload becomes more distributed, which means less operational stress on the entire organisation"

End-to-end proces analysis

The first two years of the project were devoted almost entirely to the proces analysis, architecture and the technical foundations. The goal was to ensure the end-to-end chain worked reliably at scale before a single client went live. "Previous attempts to build a new payroll engine in Belgium had limited success," Guelluy notes. "We were determined to do this properly, and that meant not rushing the fundamentals."

The engine was designed from the ground up as a cost-sharing platform that is real-time, API-driven, and modular. Scalability was non-negotiable, as the system needs to handle very high volumes of calculations in short timeframes. Precision was equally vital. "Errors are simply not an option," Guelluy says. "We’re talking about people’s salaries here. Everything has to be 100% correct, every single time."

Errors are simply not an option. We’re talking about people’s salaries here.

By 2022, the first organizations went live. Today, more than 40,000 Acerta clients run on the new engine and have done so stably for four years.

Shared Ownership

As the scope of the ambition grew, so did the thinking around how to sustain it. In 2021, Group S — another major Belgian payroll service providor — joined the initiative as a cost-sharing partner. In 2023, the collaboration was formalised into a joint venture: EverESSt, owned by Acerta, Group S and Cegeka. The initiative transitioned from a private modernization effort into a collective mission to establish the industry standard for payroll calculations.

“Cegeka has been involved from the very beginning of the trajectory, present during the architectural choices, the technical foundations, the first client rollouts.” That continuity, Guelluy argues, is one of the project's most significant assets. "The same people at Cegeka who were there at the start are still following the trajectory today. That creates consistency: in vision, in execution, in the decisions we make along the way."

Beyond development, Cegeka's role spans the full end-to-end scope: hosting, optimisation, integration, maintenance and support. And within the joint venture structure, Cegeka plays a neutral and connecting role between the payroll service providors, a position that matters when competitors are building shared infrastructure together.

Managing Complexity

Building and implementing the payroll engine brings significant coordination challenges, particularly when work is distributed across multiple hybrid onshore and nearshore teams. “Governance becomes even more critical in a product‑driven model where the solution is implemented across multiple organisations. Aligning priorities and decisions across organisations is a continuous effort,” says Reinout Van Bets, Program Manager at Cegeka.

40,000 Clients, and Counting

The results are there. Over 40,000 Acerta clients are currently processing payroll in real time on a platform that is stable, scalable, and capable of absorbing legislative complexity on the fly.

Over 40,000 Acerta clients are processing payroll in real time on a platform that is stable and capable of absorbing legislative complexity on the fly.

The road ahead is focused on expansion. In the coming months, Acerta will extend their services with additional joint labor committees, progressively opening up the platform to all of our clients.

"The ultimate goal is to make payroll processing cost-efficient for the entire sector," Guelluy says. "We want to become the Belgian standard. We have the foundation, now we keep building on it."